Question 1

Human-to-human spread of plague: Through which route is pneumonic plague transmitted directly from an infected person to another individual?

Accepted Answer

Introduction / Context: Plague, caused by Yersinia pestis, presents primarily as bubonic, septicemic, or pneumonic disease. While zoonotic transmission typically involves fleas, person-to-person spread is a critical concern in pneumonic plague outbreaks. Understanding the specific transmission route guides infection control and public health interventions. Given Data / Assumptions: The clinical form is pneumonic plague with lung involvement. Direct human-to-human spread is being asked, not animal vector transmission. We consider standard routes: inoculation, ingestion, droplet/aerosol. Concept / Approach: In pneumonic plague, bacteria are present in respiratory secretions. Coughing or close contact generates infectious droplets, which are inhaled by contacts, leading to rapid-onset pneumonia. This contrasts with bubonic plague, typically acquired through flea-borne inoculation via bites, and with ingestion, which is not the classic route for person-to-person spread. Step-by-Step Solution: Identify disease form: pneumonic indicates respiratory focus.Map to transmission: respiratory droplets in close contact.Exclude inoculation (flea/needlestick) and ingestion (food-borne) for human-to-human spread.Select droplet infection. Verification / Alternative check: Historical outbreaks show rapid propagation in crowded settings via coughing; droplet precautions (masking, isolation) interrupt chains of transmission. Why Other Options Are Wrong: Inoculation: typical for flea vectors, not from patient to patient. Ingestion: not a recognized main route for pneumonic plague spread. All of these: overly broad and incorrect for the specified clinical form. Common Pitfalls: Assuming all plague forms spread the same way; route depends on clinical syndrome and reservoir. Final Answer: droplet infection

Question 2

Enzymatic markers among Enterobacteriaceae: Which of the following genera commonly produce the enzyme β-galactosidase (detectable by ONPG tests and lactose-fermentation profiles)?

Accepted Answer

Introduction / Context: β-galactosidase hydrolyzes lactose to glucose and galactose and is central to lactose-fermentation phenotypes. In diagnostic microbiology, ONPG (o-nitrophenyl-β-D-galactopyranoside) testing helps detect β-galactosidase even when lactose permease is lacking, clarifying late or weak lactose-fermenters. Given Data / Assumptions: Genera considered: Citrobacter, Enterobacter, Serratia. Question focuses on β-galactosidase presence, not necessarily strong lactose permease activity. Standard lab conditions and conventional strains are assumed. Concept / Approach: Many Citrobacter and Enterobacter isolates are ONPG positive and lactose-fermenting. Serratia species may ferment lactose slowly or be variable, but commonly possess β-galactosidase detectable by ONPG even when lactose fermentation on plates is delayed. Therefore, across these genera, β-galactosidase production is a recognized trait. Step-by-Step Solution: Consider lactose metabolism: requires permease and β-galactosidase.Recall ONPG detects β-galactosidase independently of permease.Map each genus: Citrobacter (+), Enterobacter (+), Serratia (often ONPG + though lactose on plates may be delayed).Select the inclusive option. Verification / Alternative check: Commercial identification systems (e.g., API 20E, VITEK) include ONPG/β-gal tests that commonly score positive for these genera. Why Other Options Are Wrong: Any single-genus choice would omit other valid β-gal producers; the group-level pattern supports “all of these.” Common Pitfalls: Equating plate lactose fermentation with ONPG positivity; some strains are late fermenters due to permease deficits despite β-gal presence. Final Answer: all of these

Question 3

Zoonotic salmonellosis: Which Salmonella serovar is the most common cause of zoonotic disease in humans across diverse animal reservoirs and food sources?

Accepted Answer

Introduction / Context: Non-typhoidal Salmonella (NTS) infections are major zoonoses transmitted via contaminated animal products (poultry, eggs, meat, dairy) or direct animal contact. Among numerous serovars, a few dominate global human disease burden, making their recognition important for clinicians and public health. Given Data / Assumptions: We are focusing on zoonotic, non-typhoidal Salmonella. Human infection typically results from foodborne exposure. Common serovars include Typhimurium and Enteritidis; regional variations occur. Concept / Approach: Historically and globally, Salmonella Typhimurium has been among the most frequently isolated serovars from human gastroenteritis with broad animal reservoirs. S. Enteritidis is also prominent (notably linked to eggs), but S. Typhimurium is widely cited as the commonest zoonotic serovar across species. Step-by-Step Solution: Identify the question focus: the single most common zoonotic serovar.Compare epidemiology: Typhimurium vs Enteritidis and others.Select the best-supported global “commonest” serovar: S. typhimurium.Confirm it aligns with multi-host reservoirs and frequent human isolation. Verification / Alternative check: Surveillance reports from multiple countries repeatedly list S. Typhimurium among the top serovars in human disease, reinforcing the choice. Why Other Options Are Wrong: S. enteritidis: very common but often second to Typhimurium in many datasets. S. newport, S. Indiana: important regionally but less dominant globally. Common Pitfalls: Assuming regional spikes (e.g., egg-associated Enteritidis) define the global “commonest”; the question asks broadly across reservoirs. Final Answer: S typhimurium

Question 4

Selective media for Salmonella isolation: Which of the following culture media is specifically selective for Salmonella, aiding in primary isolation from clinical or food specimens?

Accepted Answer

Introduction / Context: Appropriate selective media increase the likelihood of recovering Salmonella from mixed flora. Different media are tailored to different genera, so choosing correctly prevents false negatives and saves lab time. Given Data / Assumptions: We compare three named media. Goal: selection for Salmonella during isolation. Standard formulations and uses are assumed. Concept / Approach: Bismuth sulphite agar (Wilson and Blair's medium) is a selective medium specifically designed for Salmonella (notably S. Typhi), incorporating brilliant green, bismuth sulphite, and other inhibitors that suppress competing flora while allowing characteristic black colonies due to H2S production. In contrast, TCBS (thiosulfate-citrate-bile salts-sucrose) is selective for Vibrio species, and generic bile salt agar selects for enterics but is not specifically tailored to Salmonella over other Enterobacteriaceae. Step-by-Step Solution: Identify which listed medium targets Salmonella: Wilson and Blair's bismuth sulphite agar.Recognize TCBS selectively isolates Vibrio, not Salmonella.Acknowledge bile salt agar as broadly selective but not specific for Salmonella.Choose the medium explicitly formulated for Salmonella. Verification / Alternative check: Diagnostic manuals recommend bismuth sulphite agar or other Salmonella-selective media (e.g., XLD, HE) rather than TCBS when Salmonella is suspected. Why Other Options Are Wrong: TCBS: optimized for Vibrio cholerae/parahaemolyticus recovery. Bile salt agar: lacks the specificity for Salmonella. All of the above: incorrect because media differ in target organisms. Common Pitfalls: Using TCBS for suspected Salmonella will miss isolates; always match medium to suspected pathogen. Final Answer: Wilson and Blair's brilliant-green bismuth sulphite agar medium

Question 5

Serology in enteric fever: In the Widal agglutination test, what is generally considered a significant diagnostic titre for O (somatic) agglutinins in a single sample, assuming an endemic setting and appropriate clinical correlation?

Accepted Answer

Introduction / Context: The Widal test measures agglutinating antibodies against Salmonella enterica serovars Typhi and Paratyphi antigens (O and H). While culture remains the gold standard, Widal is still used in resource-limited settings. Knowing typical significance thresholds helps avoid over- or under-diagnosis. Given Data / Assumptions: Endemic area with baseline titres present in the population. Single-sample interpretation (rise in paired sera is better but often unavailable). We focus on O agglutinins, which tend to rise early and indicate active infection. Concept / Approach: In many endemic regions, an O agglutinin titre of 1:100 or greater is commonly taken as significant in the right clinical context, recognizing that local baseline titres and lab-specific cutoffs vary. Lower titres (e.g., 1:50) are often non-specific; higher cutoffs (e.g., 1:150) may be too stringent in some locales. Step-by-Step Solution: Identify standard threshold used in many labs for O antibodies.Balance sensitivity and specificity in endemic settings.Select 1:100 or more as a generally significant titre (with clinical correlation).Note that paired sera showing a fourfold rise is most diagnostic. Verification / Alternative check: Local public health guidelines often publish baseline titre distributions; many cite O ≥ 1:100 as significant when supported by symptoms and epidemiology. Why Other Options Are Wrong: 1:50: too low in endemic regions; poor specificity. 1:150: may miss true positives where background levels are lower. 1:2.00: not a valid dilution expression (likely intended 1:200); overly stringent for many settings. Common Pitfalls: Relying solely on a single Widal titre without considering clinical picture, vaccination history, or alternative diagnoses. Final Answer: 1:100 or more

Question 6

Pathogenicity testing: The Sereny test (guinea pig keratoconjunctivitis model) is classically used to detect which diarrheagenic category of Escherichia coli based on invasiveness?

Accepted Answer

Introduction / Context: Diarrheagenic E. coli strains are grouped by pathogenic mechanisms: EPEC (adherence/effacement), ETEC (toxins LT/ST), EHEC/VTEC (Shiga-like toxins), and EIEC (invasion). The Sereny test historically assesses invasive capacity by inducing keratoconjunctivitis in guinea pig eyes. Given Data / Assumptions: Sereny test involves inoculating the conjunctival sac with bacteria. Positive result indicates epithelial invasion and inflammation. Animal-use caveats acknowledged; the test is mainly of historical/teaching interest. Concept / Approach: EIEC shares invasion mechanisms with Shigella (including plasmid-mediated invasion). Hence, EIEC and Shigella give positive Sereny reactions, whereas EPEC, ETEC, and EHEC/VTEC generally do not, as their pathogenesis does not require epithelial invasion in the same way. Step-by-Step Solution: Recall each E. coli pathotype mechanism.Match invasive phenotype to EIEC.Identify Sereny test as an invasion assay.Select enteroinvasive E. coli. Verification / Alternative check: Cell culture invasion assays (e.g., HeLa, Henle) and PCR for invasion genes (ipaH) provide modern confirmation parallel to the historical Sereny test. Why Other Options Are Wrong: EPEC: attachment/effacing, not invasive. ETEC: enterotoxin-mediated secretory diarrhea, noninvasive. VTEC/EHEC: toxin-mediated; invasion is not the primary mechanism. Common Pitfalls: Confusing cytotoxicity (toxins) with invasion; Sereny specifically reflects epithelial invasion causing keratoconjunctivitis. Final Answer: enteroinvasive E. coli

Question 7

Etiology of traveller's diarrhea: Which diarrheagenic Escherichia coli pathotype most commonly causes acute, watery traveller's diarrhea acquired from contaminated food or water in resource-limited settings?

Accepted Answer

Introduction / Context: Traveller's diarrhea is a common syndrome among visitors to endemic regions, characterized by sudden-onset, watery stools, cramps, and sometimes low-grade fever. Identifying the predominant etiologic agent informs prevention (food and water hygiene) and empiric therapy decisions. Given Data / Assumptions: Illness is acute and watery rather than dysenteric. Exposure is via contaminated food or water while traveling. We focus on E. coli pathotypes. Concept / Approach: ETEC is the classic cause of traveller's diarrhea. It colonizes the small intestine and produces heat-labile (LT) and/or heat-stable (ST) enterotoxins that stimulate chloride and water secretion, leading to watery diarrhea without prominent invasion or blood. EPEC causes infantile diarrhea; EHEC causes hemorrhagic colitis/HUS; EIEC is invasive and causes dysentery-like illness. Step-by-Step Solution: Match clinical pattern (watery, non-bloody) to toxin-mediated disease.Identify ETEC toxins (LT/ST) as key drivers of secretory diarrhea.Exclude invasive or hemorrhagic pathotypes.Select ETEC. Verification / Alternative check: Field studies and outbreak investigations consistently implicate ETEC as the leading cause of traveller's diarrhea across continents. Why Other Options Are Wrong: EPEC: classically infantile, attaching/effacing pathology. EIEC: invasive with fever and blood/mucus. EHEC: bloody diarrhea and risk of HUS; undercooked beef and unpasteurized foods. Common Pitfalls: Assuming antibiotics are always required; many cases are self-limited, with oral rehydration as first-line management. Final Answer: entero toxigenic E. coli (ETEC)

Question 8

Assays for Escherichia coli heat-labile enterotoxin (LT): Which of the following laboratory techniques are used to demonstrate or quantify LT activity or antigen?

Accepted Answer

Introduction / Context: ETEC produces two principal enterotoxins: heat-labile (LT), structurally and functionally similar to cholera toxin, and heat-stable (ST). Detecting LT helps confirm ETEC in clinical and research settings. Several classic bioassays and immunoassays exist for this purpose. Given Data / Assumptions: LT increases intracellular cyclic AMP in target cells. Bioassays rely on characteristic cellular responses. Immunoassays detect toxin antigen directly with antibodies. Concept / Approach: The Y1 mouse adrenal cell assay measures steroidogenesis stimulated by elevated cAMP after LT exposure. Chinese hamster ovary (CHO) cell assays detect LT-induced morphological rounding related to cAMP changes. Enzyme-linked immunosorbent assay (ELISA) uses specific antibodies to detect LT antigen quantitatively or qualitatively in samples, offering higher throughput and specificity. Step-by-Step Solution: Recognize LT mechanism: ADP-ribosylation of Gsα → adenylate cyclase activation → cAMP rise.Link mechanism to bioassay readouts (Y1 steroidogenesis, CHO rounding).Acknowledge ELISA as a sensitive antigen detection method.Since all listed tests are valid, select the inclusive option. Verification / Alternative check: Modern labs may also use GM1 ganglioside-binding ELISAs and PCR for LT genes (eltAB), which corroborate these classical assays. Why Other Options Are Wrong: Each individual option is correct but incomplete alone; the question asks which techniques are used—hence “All of these.” Common Pitfalls: Confusing LT (cAMP-mediated) with ST (cGMP-mediated, tested in suckling mouse or T84 cell assays); choose assays appropriate to the toxin. Final Answer: All of these

Question 9

Clinical biochemistry of Escherichia coli — Which of the following reactions are typically positive for E. coli in routine identification (IMViC and basic fermentation tests)?

Accepted Answer

Introduction / Context: In diagnostic microbiology, Escherichia coli is commonly identified by a standard panel of biochemical reactions. Recognizing the characteristic reactions for E. coli helps distinguish it from other Enterobacterales (formerly Enterobacteriaceae) during routine stool, urine, and blood culture workups. Given Data / Assumptions: E. coli is a lactose and glucose fermenter. The classic IMViC profile for E. coli is Indole positive, Methyl red positive, Voges–Proskauer negative, Citrate negative. The question lists three reactions individually and asks which are usually positive. Concept / Approach: Glucose fermentation is a basic metabolic trait of E. coli, producing acid that changes indicator color on media and in carbohydrate broths. The indole test detects tryptophanase activity converting tryptophan to indole, which is positive in most E. coli strains. The methyl red test detects stable acid end-products from glucose fermentation (mixed-acid pathway), also typically positive in E. coli. Therefore, the integrative answer is that all listed reactions are commonly positive. Step-by-Step Solution: Confirm glucose fermentation: acid production occurs, indicating fermentation.Indole production: add Kovac’s reagent to detect indole; a red ring indicates a positive reaction.Methyl red test: persistent red color after methyl red reagent indicates stable acid (positive). Verification / Alternative check: Pair these results with negative Voges–Proskauer and negative Simmons citrate for the typical IMViC pattern (++--). Additional confirmatory tests include lactose fermentation on MacConkey agar and oxidase negativity. Why Other Options Are Wrong: Choosing a single reaction ignores that E. coli is characteristically positive for all three listed tests. Common Pitfalls: Overlooking atypical strains or relying on a single test. Always interpret in combination with other markers (e.g., motility, lactose fermentation) for reliable identification. Final Answer: All of these

Question 10

Enteric pathogen recovery — Which of the following serve as enrichment media for isolating Salmonella from clinical or food samples?

Accepted Answer

Introduction / Context: Detecting Salmonella in stool and food requires suppressing commensals while allowing target organisms to multiply. Enrichment broths are designed to favor Salmonella growth over competing flora, improving sensitivity before plating on selective agars. Given Data / Assumptions: Tetrathionate and selenite compounds inhibit many coliforms while allowing Salmonella to grow. Brilliant green dye further suppresses Gram-positive bacteria and some Gram-negatives. The aim is pre-enrichment/enrichment prior to selective plating (e.g., XLD, HE, SS agar). Concept / Approach: Tetrathionate broth and brilliant green tetrathionate broth inhibit competing intestinal flora by using tetrathionate and dye, respectively. Selenite-F broth similarly suppresses commensals and enriches Salmonella. Because all three broths are established enrichment media used in protocols for Salmonella recovery, the correct choice is “All of these.” Step-by-Step Solution: Pre-enrich sample if required (e.g., buffered peptone water), then inoculate enrichment broth.Incubate under recommended conditions to enhance Salmonella growth.Subculture onto selective/differential plates for presumptive colonies followed by biochemical/serological confirmation. Verification / Alternative check: Method standards (e.g., food microbiology manuals) list these broths in validated workflows, confirming their role as enrichment media. Why Other Options Are Wrong: Selecting only one medium reduces recovery because different strains may prefer different enrichments; using multiple enrichments increases yield. Common Pitfalls: Skipping enrichment or over-incubating, which can allow overgrowth of competitors and reduce Salmonella recovery. Final Answer: All of these

Question 11

Shiga-like toxins — Verotoxin 1 (SLT-1) produced by certain E. coli strains is most similar in structure and action to which bacterial toxin?

Accepted Answer

Introduction / Context: Enterohemorrhagic E. coli (EHEC) produce Shiga-like (verotoxin) toxins that cause hemorrhagic colitis and can lead to hemolytic uremic syndrome. Understanding toxin homology clarifies pathogenesis and informs diagnostic tests and antitoxin strategies. Given Data / Assumptions: Verotoxin 1 (SLT-1) is produced by EHEC such as E. coli O157:H7. Shigella dysenteriae type 1 produces classic Shiga toxin. Toxins can share receptor specificity and enzymatic mechanisms. Concept / Approach: Verotoxin 1 is closely related to Shigella dysenteriae Shiga toxin, both binding the Gb3 receptor and inactivating the 60S ribosomal subunit via N-glycosidase activity, thereby halting protein synthesis. In contrast, cholera toxin and E. coli heat-labile toxin are ADP-ribosylating toxins that activate adenylate cyclase to raise cAMP; the heat-stable toxin increases cGMP via guanylate cyclase activation. Therefore, the best match for similarity is Shiga toxin. Step-by-Step Solution: Identify SLT-1 as a Shiga-like toxin with ribosome-inactivating activity.Compare mechanisms: SLT-1 and Shiga toxin share receptor and enzymatic function.Exclude enterotoxins that act via cAMP or cGMP pathways. Verification / Alternative check: Immunoassays and genetic probes often cross-detect Shiga and Shiga-like toxins due to homology, supporting the similarity. Why Other Options Are Wrong: Cholera toxin / heat-labile E. coli toxin: stimulate cAMP, not ribosome inactivation. Heat-stable toxin: increases cGMP, distinct mechanism and structure. Common Pitfalls: Confusing clinical syndromes: watery secretory diarrhea (cAMP/cGMP toxins) versus bloody diarrhea and HUS (Shiga-like toxins). Final Answer: shiga toxin

Question 12

Virulence of Yersinia pestis — Which of the following features contribute to the pathogenicity of the plague bacillus?

Accepted Answer

Introduction / Context: Yersinia pestis, the agent of plague, possesses multiple virulence determinants that enable flea-borne transmission, immune evasion, and rapid systemic spread. Recognizing these factors supports laboratory suspicion and epidemiologic control. Given Data / Assumptions: V and W antigens (including LcrV) are associated with type III secretion and immune modulation. F1 envelope (capsular) antigen promotes antiphagocytic protection at mammalian temperatures. Pigmentation (Pgm) phenotype on haemin-containing media correlates with iron acquisition systems. Concept / Approach: All listed features are linked to Y. pestis virulence. The F1 antigen forms a temperature-regulated capsule, prominent at 37°C. V and W antigens relate to secretion apparatus and immune manipulation. The ability to form pigmented colonies on haemin media reflects iron uptake systems important for in vivo survival. Together, they underpin pathogenicity, making “All of these” correct. Step-by-Step Solution: Associate each antigen/phenotype with a known virulence function.Note temperature regulation of F1 (capsule visible at 37°C).Conclude that all features contribute to pathogenicity. Verification / Alternative check: Mutants lacking these determinants often show attenuated virulence in experimental models, corroborating their roles. Why Other Options Are Wrong: Picking a single factor ignores the multifactorial virulence of Y. pestis. Common Pitfalls: Assuming pigmentation is merely cosmetic; in Y. pestis it signals iron acquisition competence. Final Answer: All of these

Question 13

Capsule expression in plague — In cultures of Yersinia pestis, the antiphagocytic F1 capsule is best demonstrated at which incubation temperature?

Accepted Answer

Introduction / Context: Yersinia pestis expresses virulence factors in a temperature-dependent manner, reflecting its dual life in ectothermic fleas and warm-blooded mammals. Correct incubation temperature is crucial for demonstrating the F1 capsule, a key laboratory clue for presumptive identification. Given Data / Assumptions: F1 antigen is the temperature-regulated capsular antigen. Expression is minimal in flea-associated temperatures (~26–28°C) and upregulated at mammalian temperatures. Laboratory demonstration uses appropriate media and incubation conditions. Concept / Approach: The F1 capsule is optimally expressed at 37°C, consistent with mammalian host temperature, aiding immune evasion. At lower temperatures (e.g., 25–29°C) typical of the flea vector, F1 expression is reduced, fitting transmission ecology. Therefore, the best temperature to demonstrate the capsule in culture is 37°C. Step-by-Step Solution: Recall temperature regulation of virulence genes in Y. pestis.Match capsule expression peak to mammalian temperature (37°C).Select 37°C as the correct answer. Verification / Alternative check: Serology or latex agglutination for F1 antigen is typically performed on cultures grown at 37°C, confirming antigen presence. Why Other Options Are Wrong: 25–29°C: vector-phase temperature with reduced F1 expression. 45°C: not suitable for growth; heat stress would inhibit culture. Common Pitfalls: Attempting capsule detection from cultures grown at room temperature may yield false-negative results. Final Answer: 37°C

Question 14

Community-acquired urinary tract infection — The most common etiological agent in otherwise healthy outpatients is:

Accepted Answer

Introduction / Context: Urinary tract infections (UTIs) are among the most frequent bacterial infections in the community, especially in women. Accurate empirical therapy depends on knowing the most likely pathogen before culture results return. Given Data / Assumptions: Community-onset UTI in otherwise healthy adults. No complicating factors such as catheters or recent hospitalization (which shift etiology). Standard epidemiology of uropathogens applies. Concept / Approach: Escherichia coli causes the majority of uncomplicated community-acquired UTIs due to virulence factors like P fimbriae, adhesins, and the ability to colonize the periurethral area from gut reservoirs. While Klebsiella, Proteus, and Citrobacter can cause UTIs, their prevalence is significantly lower in community settings compared with E. coli. Step-by-Step Solution: Identify setting: community-acquired, uncomplicated infection.Recall predominant pathogen: uropathogenic E. coli (UPEC).Select “Escherichia coli” as the most common agent. Verification / Alternative check: Large surveillance studies consistently report E. coli as the leading cause of outpatient UTIs, supporting this choice. Why Other Options Are Wrong: Klebsiella/Proteus/Citrobacter: occasional causes, more frequent in complicated or healthcare-associated infections. Common Pitfalls: Assuming the same distribution applies to catheter-associated or nosocomial UTIs; those settings have broader etiologies and resistance patterns. Final Answer: Escherichia coli

Question 15

Etiology of plague — Which organism is the causative agent of plague in humans?

Accepted Answer

Introduction / Context: Plague is a severe zoonotic infection historically responsible for pandemics. Correctly identifying the causative organism is fundamental for diagnosis, public health reporting, and outbreak control. Given Data / Assumptions: The question asks specifically for agents that can cause plague. Only one of the listed organisms is known to cause plague. We assume standard clinical microbiology knowledge. Concept / Approach: Yersinia pestis is the etiologic agent of plague (bubonic, septicemic, and pneumonic forms). It is transmitted primarily via flea vectors from rodent reservoirs or via aerosols in pneumonic plague. Proteus mirabilis and Pseudomonas aeruginosa are important pathogens in other contexts (e.g., urinary infections and opportunistic healthcare-associated infections) but do not cause plague. Step-by-Step Solution: Match the disease “plague” to its historical and scientific agent.Exclude unrelated Gram-negative rods (Proteus, Pseudomonas) lacking epidemiologic linkage to plague.Select Yersinia pestis as the sole correct causative agent. Verification / Alternative check: Laboratory identification includes bipolar (“safety-pin”) staining, growth characteristics, and detection of F1 antigen; epidemiology aligns with flea-rodent cycles. Why Other Options Are Wrong: Proteus mirabilis: uropathogen with swarming motility; not a plague agent. Pseudomonas aeruginosa: opportunistic pathogen; unrelated to plague. All of these: incorrect because only Y. pestis causes plague. Common Pitfalls: Confusing “Yersinia” species; plague is specifically due to Y. pestis, not to other Enterobacterales. Final Answer: Yersinia pestis

Question 16

Shigella biochemistry — Which Shigella species is typically negative for fermentation of mannitol, helping in laboratory differentiation?

Accepted Answer

Introduction / Context: Shigella species are biochemically similar but can be differentiated by certain sugar fermentations. Mannitol fermentation is a classic discriminator in clinical laboratories for presumptive species-level identification. Given Data / Assumptions: Shigella genus includes S. dysenteriae, S. flexneri, S. boydii, and S. sonnei. Most Shigella ferment mannitol; one species is classically mannitol-negative. We assume standard interpretations on differential media. Concept / Approach: S. dysenteriae is typically mannitol non-fermenting, while S. flexneri, S. boydii, and S. sonnei generally ferment mannitol. This property, along with other biochemical and serologic differences, aids rapid differentiation pending serotyping. Step-by-Step Solution: List Shigella species and recall typical mannitol reaction.Identify S. dysenteriae as the exception (mannitol-negative).Choose “S dysenteriae” as the correct answer. Verification / Alternative check: On appropriate media, mannitol fermentation results in indicator color change for most species except S. dysenteriae; serogrouping confirms identity. Why Other Options Are Wrong: S. flexneri, S. boydii, S. sonnei: generally mannitol-positive, not matching the question’s criterion. Common Pitfalls: Relying solely on a single biochemical test; occasional strain variation exists, so confirm with serology or molecular assays. Final Answer: S dysenteriae

Question 17

Motility of spirochaetes — Which statement best describes the motility exhibited by spirochaetes under the microscope?

Accepted Answer

Introduction / Context: Spirochaetes (e.g., Treponema, Borrelia, Leptospira) are thin, helical bacteria that move using endoflagella (axial filaments) located within the periplasmic space. Their unique structure results in distinctive motility patterns that aid in microscopic recognition. Given Data / Assumptions: Observation is typically made using dark-field or phase-contrast microscopy. Endoflagella anchor at each end and overlap around the protoplasmic cylinder. Movement reflects rotation of the axial filament complex relative to the cell body. Concept / Approach: The endoflagella-driven mechanism generates multiple observable motions: a corkscrew-like (rotatory) movement that helps penetrate viscous media; flexion and extension (bending) of the helical body; and translatory (forward) motion as the cell progresses. Because spirochaetes exhibit all three, the comprehensive answer is “all of the above.” Step-by-Step Solution: Identify endoflagella as the motor apparatus producing torque and bending.Relate torque to helical rotation (corkscrew) and body flexion/extension.Acknowledge forward translocation as the net result in liquid or gel-like environments. Verification / Alternative check: Dark-field videos of Treponema pallidum show all three components of movement, validating this description. Why Other Options Are Wrong: Choosing a single mode ignores the composite nature of spirochaete motility driven by axial filaments. Common Pitfalls: Confusing spirochaete motility with flagellated rods; classical external flagella produce different swimming patterns. Final Answer: all of the above

Question 18

Salmonella enterica serotypes — Which of the following serotypes commonly cause acute gastroenteritis in humans?

Accepted Answer

Introduction / Context: Nontyphoidal Salmonella (NTS) serotypes are major causes of foodborne gastroenteritis worldwide. Recognizing frequent serotypes helps in outbreak investigations and public health surveillance. Given Data / Assumptions: We focus on gastroenteritis (diarrhea, cramps, fever) rather than typhoid fever. Salmonella enteritidis, S. typhimurium, and S. newport are well-known NTS serotypes. Transmission typically occurs via contaminated poultry, eggs, meat, produce, or cross-contamination. Concept / Approach: All listed serotypes are implicated in human gastroenteritis outbreaks. S. enteritidis is frequently associated with eggs and poultry; S. typhimurium is a common broad host-range serotype; S. newport causes sporadic illness and outbreaks linked to produce and animal products. Therefore, “All of these” is correct. Step-by-Step Solution: Identify the disease entity: acute gastroenteritis (nontyphoidal Salmonella).Match each listed serotype to documented human gastroenteritis cases.Conclude that all listed serotypes can cause gastroenteritis. Verification / Alternative check: Public health reports routinely cite these serotypes among the top causes of Salmonella gastroenteritis, supporting the answer. Why Other Options Are Wrong: Choosing a single serotype ignores the epidemiologic diversity of NTS infections. Common Pitfalls: Confusing typhoidal serovars (e.g., S. Typhi, S. Paratyphi) with NTS; the question explicitly concerns gastroenteritis. Final Answer: All of these

Question 19

Thermal inactivation of enteric pathogens (food microbiology/public health): At a constant moist-heat temperature of 55 °C, which of the following exposure times would most reliably and readily kill Salmonella species in contaminated materials under…

Accepted Answer

Introduction / Context: Salmonella species are common enteric pathogens implicated in food-borne illness and laboratory-acquired infections. Understanding time–temperature combinations that inactivate Salmonella helps in validating sanitation steps, pasteurization surrogates, and biosafety procedures. This item asks which exposure time at a sub-pasteurization temperature (55 °C) will reliably kill Salmonella under typical conditions. Given Data / Assumptions: Moist heat is more effective than dry heat at the same temperature because water facilitates protein denaturation. The set-point temperature is fixed at 55 °C (below classic milk pasteurization regimens). Goal is reliable kill (“readily kill”), not borderline reduction. Matrix effects (fat, protein) and initial load can prolong thermal death times; we assume conservative practice. Concept / Approach: Thermal lethality depends on D-values (time for a 1 log10 reduction at a given temperature) and z-values (temperature change needed to change D by tenfold). At 55 °C, D-values for Salmonella are longer than at 60–72 °C. Therefore, longer holding times are needed for dependable inactivation, especially in protective food matrices or contaminated lab fluids. Choosing the longest offered exposure at 55 °C best satisfies “readily kill.” Step-by-Step Solution: Fix the temperature at 55 °C; only time varies. Recall that pasteurization benchmarks (e.g., 63 °C for 30 min) exceed both this temperature and most listed times. Account for safety margin due to matrix protection and higher initial loads. Select the longest exposure: 55 °C for 1 hour as the most reliable choice. Verification / Alternative check: Empirical studies show marked decreases in D55 for Salmonella with time; operational guidance often uses longer holds at sub-pasteurization temperatures to ensure adequate lethality when higher temperatures are impractical. Why Other Options Are Wrong: 55 °C for 10–30 minutes may reduce counts but is less dependable across matrices and loads; 50 °C for 30 minutes is even less lethal and not a standard inactivation step. Common Pitfalls: Assuming that any brief heating ensures sterility; at modest temperatures, organisms survive longer, and fat/protein can protect them. Final Answer: Moist heat at 55 °C for 1 hour.

Question 20

Epidemiology of plague – dominant transmission route for bubonic disease: In human outbreaks of bubonic plague caused by Yersinia pestis, which route is classically responsible for transmission to people?

Accepted Answer

Introduction / Context: Plague presents in bubonic, septicemic, and pneumonic forms. Identifying the major transmission route for bubonic plague is essential for vector control and public health responses in endemic settings and during epizootics among rodents. Given Data / Assumptions: Agent: Yersinia pestis. Vectors: fleas parasitizing rodents, especially Xenopsylla cheopis. Clinical form in question: bubonic plague (painful buboes, regional lymphadenitis). Concept / Approach: Bubonic plague follows inoculation of Y. pestis into skin via bites of infected rodent fleas. The bacteria spread via lymphatics to draining nodes, producing buboes. Inhalation primarily causes primary pneumonic plague; ingestion is not a standard route for bubonic disease. Hence the vector bite is the classical route for bubonic cases. Step-by-Step Solution: Match clinical syndrome (bubonic) with known pathogenesis (lymphatic spread after skin inoculation). Identify the arthropod vector: rat flea. Exclude inhalation (pneumonic plague) and ingestion (nonclassical). Choose flea bite as the correct route. Verification / Alternative check: Historic and modern outbreaks correlate human bubonic cases with epizootics in rats and increased flea indices; vector control curbs transmission. Why Other Options Are Wrong: Inhalation relates to pneumonic plague; ingestion is not a typical route; “all of these” is overly inclusive for the bubonic form. Common Pitfalls: Conflating bubonic with pneumonic plague transmission dynamics. Final Answer: Rat flea (Xenopsylla cheopis) bite.

Spirochacter and Enterobacteriaceac Questions