Lab 12

Which disinfectants and antiseptics are generally the most effective at inhibiting bacterial growth, and how do gram-negative and gram-positive bacteria compare?

The most effective are usually 10% bleach and Lysol, which are strong, broad-spectrum disinfectants that give the largest zones of inhibition. 70% isopropanol is next (alcohols work best around 60–70% because the water helps the alcohol get inside and denature proteins).

Gram-positive bacteria (S. epidermidis, S. mutans, B. cereus) are generally inhibited more because they have no outer membrane.

Gram-negative bacteria (E. coli, P. fluorescens) are generally more resistant because the outer membrane blocks many chemicals, and Pseudomonas is especially resistant. Endospores (B. cereus) resist most disinfectants even when the vegetative cells are killed.

Which disinfectants and antiseptics are generally the least effective at inhibiting bacterial growth, and why?

The least effective are usually hand soap and 10% ethanol. Hand soap removes microbes by washing them away instead of killing them, and 10% ethanol is too dilute to denature proteins. 95% ethanol also does worse than 70% because it coagulates proteins on the cell surface, forming a barrier that blocks entry. Alcohols evaporate quickly from the disk. Hydrogen peroxide is weaker against catalase-positive organisms (Staphylococcus, Bacillus, Pseudomonas, E. coli) because catalase breaks it down into water and oxygen, while catalase-negative S. mutans is more susceptible.

Which antibiotics are generally the most effective at inhibiting growth of gram-negative and gram-positive bacteria?

Broad-spectrum antibiotics are generally the most effective at inhibiting the growth of both gram-negative and gram-positive bacteria because they can target both groups.

The primary examples emphasized in microbiology textbooks include:

• Tetracyclines (e.g., doxycycline)

• Aminopenicillins (e.g., ampicillin, amoxicillin)

• Fluoroquinolones (e.g., ciprofloxacin)

• Carbapenems (e.g., imipenem)

Which antibiotics are generally the least effective at inhibiting bacterial growth, and why?

The least effective are usually penicillin G and erythromycin against gram-negatives (E. coli, P. fluorescens), because the outer membrane keeps these large drugs from reaching their targets. Streptomycin often does poorly against streptococci like S. mutans, because aminoglycosides need oxygen-driven uptake into the cell. P. fluorescens is typically the most resistant overall, since its outer membrane and efflux pumps block many drug

What is disinfection, and what are disinfectants?

Disinfection (process)is the elimination of most or all microorganisms from an environment using chemicals called disinfectants (Lysol, bleach, kitchen and bathroom cleaners).

Disinfectants (tools) are not for use on human tissue, and they may not kill endospores, so they do not achieve sterility. (Lysol, bleach, kitchen and bathroom cleaners).

What is antisepsis, and what are antiseptics?

Antisepsis is using a chemical on human tissue to prevent infection.

Antiseptics (70% isopropanol, mouthwash, boric acid, iodine, hydrogen peroxide) are safe on living tissue, kill only vegetative cells (not endospores), and do not sterilize.

What is sterilization, and how does it differ from disinfection?

Sterilization is the removal or destruction of all microbes in an environment, killing both vegetative cells and endospores.

Disinfection does not guarantee that, since it may leave endospores alive.

What is the difference between microbicidal and microbistatic agents?

Microbicidal agents kill microorganisms.

Microbistatic agents only inhibit their growth, and the microbes can grow again once the agent is removed.

What factors must be considered when choosing a disinfectant or antiseptic?

(1) What microbes are likely present, since spores, capsules, and acid-fast cells need harsher treatment. (

2) How many microbes there are and how fast the agent works, since a high load or slow agent needs longer exposure.

(3) The environmental conditions, since dirt, grease, or blood slow killing.

(4) The material being treated, since it may be sensitive to heat, chemicals, pH, or moisture.

(5) Whether the item needs full sterilization or just fewer microbes.

How do you make a confluent lawn of bacteria on a plate?

Dip a sterile swab in the liquid culture and swab the entire plate surface, rotate the plate about 120° and swab again, then rotate another 120° and swab a third time. This covers the plate evenly so any zone of inhibition is clear.

How is the disinfectant/antiseptic disk test done?

Divide the bottom of the TSA plate into 6 labeled sections. Use forceps to dip a sterile filter paper disk into the agent, drain the excess against a paper towel, and place it in the center of its section. Flame the forceps between disks, then incubate at 37°C.

What is the Kirby-Bauer method?

The Kirby-Bauer method is a test for the susceptibility of an organism to antibiotics (or disinfectants/antiseptics).

The organism is spread evenly over an agar plate, antibiotic-impregnated disks are placed on the surface, and the plate is incubated. The drug diffuses out and forms a concentration gradient, highest at the disk and lowest at the edge.

What is a zone of inhibition (ZOI), and what does its size tell you?

The zone of inhibition is the clear area around a disk where there is no growth. It is measured to the nearest mm. A larger zone generally means the organism is more sensitive, and the zone size is influenced by the organism's sensitivity and where the antibiotic reaches its minimum inhibitory concentration (MIC).

What is the minimum inhibitory concentration (MIC)?

The MIC is the lowest concentration of an antimicrobial that stops growth. The edge of the zone of inhibition is where the drug concentration drops to the MIC.

Why do different antibiotics need different zone sizes to count as susceptible?

Drugs diffuse at different rates because of differences in molecular weight and polarity. Some drugs need a very large zone to count as susceptible, while for others a small zone is enough, so each drug is compared against its own table.

Why is Mueller-Hinton agar used for Kirby-Bauer testing?

It is the standard medium, so diffusion is consistent and the contents of the medium do not neutralize the antimicrobial drug. The agar must also be a uniform thickness, and disks must contain the same amount of drug and not be outdated.

What is the zone of inhibition guide for interpreting results (resistant, intermediate, sensitive)?

Chloramphenicol (C30): R ≤12 mm, I 13–17, S ≥18.

Tetracycline (TE-30): R ≤14, I 15–18, S ≥19.

Levofloxacin (LVX-5): R ≤15, I 16–20, S ≥21.

Streptomycin (S-10): R ≤6, I 7–9, S ≥10.

Erythromycin (E-15): R ≤13, I 14–22, S ≥23.

Penicillin G (P-10): R ≤14, I 15–21, S ≥22.

Gentamycin (GM-10): R ≤12, I 13–14, S ≥15.

Sulfamethoxazole & trimethoprim (SXT/TMP5): R ≤12, I 13–16, S ≥17.

What does it mean for antibiotics to be selectively toxic, and what do they target?

Selectively toxic means the antibiotic harms the pathogen's cells and not the human host. They target bacterial-specific structures and processes: the cell membrane, cell wall synthesis and maintenance, DNA replication, transcription, translation, and folic acid biosynthesis.

Why is antibiotic susceptibility tested before treatment?

Not all antibiotics work on all bacteria, because bacterial species differ genetically and in their cellular processes, so testing shows which drug will work.

What is the correct way to say that a bacterium is resistant or susceptible?

Resistant and susceptible describe the bacterial culture, not the drug.

Say "the organism is resistant (or susceptible) to the antimicrobial drug," not "the drug is resistant to the organism."

Which disinfectants or antiseptics were most effective at inhibiting growth, and which were least effective?

This depends on your plates, so use your measured zones first.

The usual pattern is that 10% bleach and Lysol give the largest zones, 70% isopropanol works better than 95% ethanol (the water in 70% helps it get inside and denature proteins), and hand soap and 10% ethanol give small or no zones because they remove microbes mechanically or are too dilute.

Gram-negative organisms, especially Pseudomonas, are often more resistant than gram-positive ones because of their outer membrane.

What are the limitations of the Kirby-Bauer test for disinfectants and antiseptics, and how accurate are the results?

The method is standardized for antibiotics, not disinfectants.

Limitations: the amount of agent soaked into each disk varies, TSA is used instead of Mueller-Hinton, there are no standard zone cutoffs for disinfectants, different chemicals diffuse at different rates (and alcohols evaporate), the zone does not show whether an agent kills or only inhibits, and the zones are measured by ruler on a small sample. Results are good for rough comparison between agents, but not highly accurate or precise.

Which antibiotics were most effective and least effective at inhibiting the gram-negative and gram-positive bacteria?

This depends on your plates, so measure the zones and compare them with the S/I/R table.

The typical pattern is that penicillin G and erythromycin work better on gram-positives, since the gram-negative outer membrane blocks them. Broad-spectrum drugs (tetracycline, chloramphenicol, gentamycin, levofloxacin) tend to be effective on both. Pseudomonas fluorescens is often resistant to many antibiotic