Phenol (carbolic acid) – Threshold for killing: At approximately what concentration does phenol become bactericidal against vegetative bacteria under standard conditions?

Introduction / Context:
Phenol (carbolic acid) is a historical benchmark disinfectant. Although modern practice favors safer phenolic derivatives and other agents, understanding its concentration-response relationship is foundational for interpreting the classic “phenol coefficient” concept and the transition from bacteriostatic to bactericidal activity.

Given Data / Assumptions:

• Lower concentrations of phenol tend to be bacteriostatic (growth-inhibitory).
• Higher concentrations are bactericidal to vegetative cells under appropriate contact times.
• Spores are far more resistant than vegetative bacteria.

Concept / Approach:
Historically, around 1% phenol is cited as the threshold at which reliable killing of many vegetative bacteria occurs within practical contact times, while 0.5% is often merely inhibitory. Above 1%, phenol’s lethality increases, though toxicity and material compatibility limit use. The question targets this commonly taught threshold concentration.

Step-by-Step Solution:
1) Consider concentration-dependent effects: 0.25–0.5% → mostly static; ~1% → cidal for vegetative forms.2) Recognize that sporicidal action requires much stronger conditions and is not implied here.3) Identify the best single answer aligned with classic teaching: 1%.4) Select 1% as the bactericidal concentration for vegetative cells under standard testing.

Verification / Alternative check:
Early disinfection literature and teaching manuals commonly describe 1% phenol as bactericidal to many vegetative organisms, forming the basis for comparative phenol coefficient testing of other disinfectants.

Why Other Options Are Wrong:

• 0.25% and 0.5%: typically bacteriostatic levels.
• 0.75%: below the commonly cited bactericidal threshold.
• 2% only: killing can occur at lower concentrations given adequate time; 2% is not required for vegetative cells.

Common Pitfalls:
Assuming sporicidal activity at 1% phenol; ignoring that contact time, temperature, and organic load modify outcomes but do not negate the standard threshold concept.

Final Answer:
1%.