Question 1

Enteric bacteriology – H2S production by Salmonella: Which Salmonella serotype listed is classically non–hydrogen sulfide (H2S) producing on routine differential media, aiding presumptive identification in the laboratory?

Accepted Answer

Introduction / Context: Production of hydrogen sulfide (H2S) on media such as triple sugar iron (TSI) agar or Hektoen enteric agar is a useful screening trait for enteric pathogens. Most Salmonella produce H2S, but notable exceptions help refine preliminary identification. Given Data / Assumptions: Most Salmonella serotypes are H2S positive (blackening on media). S. Paratyphi A and S. choleraesuis are classically H2S negative. H2S phenotype supports, but does not by itself confirm, species/serotype identification. Concept / Approach: Recognizing the exceptions prevents misclassification. Non–H2S-producing Salmonella can resemble Shigella colonies on some media, so additional biochemical and serologic tests are required. The well-taught pairing of S. Paratyphi A and S. choleraesuis as H2S negative is a common examination point. Step-by-Step Solution: Recall the rule: “Most Salmonella are H2S positive.” Identify exceptions: S. Paratyphi A and S. choleraesuis. Select the combined option listing both exceptions. Plan confirmatory serology if encountered in practice. Verification / Alternative check: Bench manuals highlight these serotypes as non–H2S producers, advising careful differentiation from Shigella. Why Other Options Are Wrong: Choosing one alone is incomplete; “none” ignores known exceptions; S. Typhi often yields weak but present H2S. Common Pitfalls: Overreliance on H2S alone for identification; always corroborate with antigenic typing and additional biochemicals. Final Answer: Both (a) and (b).

Question 2

Diagnostic pigment – prodigiosin production: Which bacterial genus is renowned for producing the red pigment prodigiosin, often yielding striking red colonies on agar at room temperature?

Accepted Answer

Introduction / Context: Pigment production can be a useful phenotypic clue in bacteriology. The red pigment prodigiosin is historically linked to certain environmental and opportunistic organisms, aiding presumptive identification from unusual colony colors. Given Data / Assumptions: Prodigiosin is a red, tripyrrolic pigment. Incubation at lower temperatures (e.g., 22–28 °C) enhances pigment expression. Genus-level association is sought. Concept / Approach: Serratia marcescens and related Serratia species can produce prodigiosin, causing vivid red colonies that have even inspired historical legends. While pigment production can vary with strain and conditions, Serratia remains the classic answer in medical microbiology. Step-by-Step Solution: Identify the pigment (prodigiosin) and its signature red hue. Link pigment to its canonical genus, Serratia. Exclude other Enterobacterales genera that lack this hallmark pigment. Select Serratia. Verification / Alternative check: Laboratory atlases display red colonies of Serratia marcescens under room-temperature incubation, confirming the association. Why Other Options Are Wrong: Hafnia, Enterobacter, Citrobacter, and Klebsiella generally do not produce prodigiosin as a defining trait. Common Pitfalls: Expecting pigment at 35–37 °C; many Serratia strains pigment better at cooler temperatures. Final Answer: Serratia.

Question 3

Isolation strategy – enrichment for Shigella: Which of the following broth media serves as an enrichment medium specifically for isolating Shigella from stool specimens?

Accepted Answer

Introduction / Context: Shigella are fastidious enteric pathogens with low infectious dose. Unlike Salmonella and Vibrio, Shigella typically lack a widely adopted enrichment broth; instead, labs rely on prompt processing and selective/differential agars to recover them from stool. Given Data / Assumptions: Alkaline peptone water enriches Vibrio cholerae. Selenite F and tetrathionate broths enrich Salmonella. Question asks specifically for Shigella enrichment among the listed choices. Concept / Approach: Because Shigella may be inhibited by the same conditions that favor Salmonella or Vibrio, many protocols avoid enrichment and instead plate directly onto selective/differential media such as XLD, MacConkey, Hektoen enteric, or SS agar. Therefore, among the options given, none correctly represents a Shigella enrichment. Step-by-Step Solution: Map each medium to its intended target group. Observe the absence of a Shigella-specific enrichment among the listed choices. Select “None of these.” Remember to use timely plating on selective media in practice. Verification / Alternative check: Standard texts emphasize direct plating and rapid transport for Shigella recovery; some labs may use GN broth, but it is not among the options provided. Why Other Options Are Wrong: Alkaline peptone water targets Vibrio; selenite F and tetrathionate target Salmonella, not Shigella. Common Pitfalls: Assuming all enteric pathogens have standard enrichments; Shigella commonly does not. Final Answer: None of these (Shigella is usually not enriched; use selective/differential plating).

Question 4

Systematics – members of the family Spirochaetaceae: Which genera listed below are classified within the family Spirochaetaceae and share the characteristic flexible, helical, motile morphology?

Accepted Answer

Introduction / Context: Spirochaetes are slender, helically coiled bacteria exhibiting unique motility by periplasmic endoflagella (axial filaments). Recognizing genera within Spirochaetaceae supports clinical reasoning for diseases such as syphilis and Lyme borreliosis. Given Data / Assumptions: Family Spirochaetaceae includes several medically and environmentally relevant genera. Treponema (e.g., T. pallidum), Borrelia (e.g., B. burgdorferi), and Spirochaeta are prototypical members. Question tests taxonomic awareness at the genus level. Concept / Approach: All three listed genera display the spirochaete body plan and axial filament-driven motility, placing them within Spirochaetaceae. Their clinical relevance spans sexually transmitted infections (Treponema), vector-borne diseases (Borrelia), and environmental niches (Spirochaeta). Step-by-Step Solution: Identify each genus as a known spirochaete. Confirm they belong to Spirochaetaceae in classical microbiology classification. Select the inclusive answer “All of these.” Recall hallmark motility: periplasmic flagella. Verification / Alternative check: Microscopy of spirochaetes shows characteristic corkscrew motion; clinical testing often uses dark-field or serology due to culture difficulty. Why Other Options Are Wrong: Single selections omit other true members; “none” contradicts well-established taxonomy. Common Pitfalls: Confusing Spirochaetaceae with morphologically similar but taxonomically separate curved or helical bacteria (e.g., Campylobacter, Helicobacter). Final Answer: All of these.

Question 5

Shigella hardiness – environmental resistance: Among the Shigella species listed, which is relatively more resistant to adverse environmental conditions (e.g., survival outside the host), making it a common cause in developed regions?

Accepted Answer

Introduction / Context: Shigellosis is caused by four major species: S. dysenteriae, S. flexneri, S. boydii, and S. sonnei. Differences in environmental survival and epidemiology influence which species predominates in certain regions and outbreak settings. Given Data / Assumptions: S. sonnei is often predominant in developed countries and displays relatively better survival in external environments compared with some other species. S. dysenteriae type 1 is highly virulent but less environmentally robust. Environmental hardiness can affect transmission via fomites or low-level contamination. Concept / Approach: Epidemiological data link S. sonnei to daycare, institutional, and community outbreaks in developed settings, consistent with better tolerance to adverse conditions and low infectious dose. Recognizing this pattern helps prioritize public health interventions. Step-by-Step Solution: Compare known survival profiles among Shigella species. Identify S. sonnei as relatively more resilient ex vivo. Select S. sonnei accordingly. Note that strict hygiene and rapid case identification are essential. Verification / Alternative check: Outbreak investigations repeatedly find S. sonnei in higher-income settings with longer environmental persistence than S. dysenteriae in similar scenarios. Why Other Options Are Wrong: S. dysenteriae is notorious for severity (Shiga toxin) but not hardiness; S. flexneri and S. boydii predominate elsewhere and are generally less environmentally tolerant than S. sonnei. Common Pitfalls: Equating virulence with environmental resistance; they are distinct traits. Final Answer: Shigella sonnei.

Spirochacter and Enterobacteriaceac Questions