Antimicrobial Susceptibility Testing and Quality Control

Flashcards covering antimicrobial susceptibility testing methodologies, standardization criteria, troubleshooting, quality control, and bacterial resistance mechanisms based on lecture notes.

Is the antimicrobial dilution method quantitative or qualitative, and what value does it produce?

The dilution method is quantitative because it produces a numerical minimum inhibitory concentration (MIC) value, which is then categorized as susceptible, intermediate, or resistant.

Is the disc diffusion (Kirby-Bauer) test quantitative or qualitative?

Disc diffusion is a qualitative test because it directly categorizes an isolate as susceptible, intermediate, or resistant based on zone of inhibition diameters.

What are the standard plate dimensions used for disc diffusion susceptibility testing?

Disc diffusion utilizes Mueller-Hinton agar plates that are $150\text{ millimeters}$ ($150\text{ mm}$) in diameter.

What is the definition of a breakpoint in antimicrobial susceptibility testing?

A breakpoint refers to specific antimicrobial concentrations (or zone diameters) that define whether an organism isolate is classified as resistant, intermediate, or susceptible.

What standard turbidity must be met when preparing a broth inoculum for susceptibility testing?

The inoculum must be standardized to a $0.5$ McFarland turbidity standard.

How does using an inoculum that is too turbid affect disc diffusion testing results?

An inoculum that is too turbid decreases the zone of inhibition size, resulting in false resistance.

How does using an inoculum that is too light affect disc diffusion testing results?

An inoculum that is too light increases the zone of inhibition size, resulting in false susceptibility, which can lead to treatment failure.

What culture age and growth phase are required for accurate antimicrobial susceptibility testing?

An $18\text{ to }24\text{ hour}$ pure culture in the logarithmic (log) growth phase must be used.

Why does using a $36\text{ hour}$ old bacterial culture alter susceptibility testing results?

A $36\text{ hour}$ old culture has reached the death or decline phase, which increases zone sizes and causes false susceptibility because many bacteria are no longer viable.

What is the recommended temperature for long-term storage of antimicrobial discs?

Antimicrobial discs must be stored long-term at $-20\text{ }^\bullet\text{C}$.

What effect does using improperly stored or expired antimicrobial discs have on zone sizes?

The antimicrobial loses potency, leading to smaller zones of inhibition and false resistance.

What is the standard incubation temperature for disc diffusion testing?

The standard incubation temperature is $35\text{ }^\bullet\text{C}$.

How does incubating disc diffusion plates at $30\text{ }^\bullet\text{C}$ or $42\text{ }^\bullet\text{C}$ impact zone size results?

Bacteria grow more slowly outside their optimal $35\text{ }^\bullet\text{C}$ temperature, producing larger zones of inhibition and false susceptibility.

What are the optimal growth temperatures for Yersinia species and Campylobacter species?

The optimal growth temperature is $30\text{ }^\bullet\text{C}$ for Yersinia species and $42\text{ }^\bullet\text{C}$ for Campylobacter species.

What is the standard concentration of calcium cations in Mueller-Hinton agar?

The standard calcium cation concentration in Mueller-Hinton agar is $35\text{ mg/L}$.

How do elevated concentrations of divalent cations in Mueller-Hinton agar affect specific antibiotics?

Excess calcium and magnesium decrease the activity of aminoglycosides (e.g., gentamicin) and tetracyclines, resulting in smaller zone sizes and false resistance.

What is the standard depth of media for Mueller-Hinton agar plates, and how do variations affect results?

The standard depth is $4\text{ mm}$. Media that is too deep produces smaller zones (false resistance), while media that is too thin produces larger zones (false susceptibility).

How should swarming of Proteus species into the zone of inhibition be measured?

The thin veil of swarming growth should be ignored, and the diameter of the obvious main zone of inhibition should be measured.

What media and incubation conditions are required for testing fastidious Haemophilus species?

Haemophilus Test Medium (HTM) containing X and V factors is required, incubated at $35\text{ }^\bullet\text{C}$ in $5\text{\text{\text{\text{\%}}}}\text{ CO}_2$ (capnophilic atmosphere).

What media and incubation conditions are required for testing fastidious Streptococcus species?

Mueller-Hinton agar supplemented with $5\text{\text{\text{\text{\%}}}}$ sheep blood is required, incubated at $35\text{ }^\bullet\text{C}$ in $\text{CO}_2$ for $20\text{ to }24\text{ hours}$.

Why does oxacillin resistance in Staphylococcus aureus indicate methicillin resistance (MRSA)?

Oxacillin and methicillin belong to the same class of penicillinase-resistant penicillins; resistance to oxacillin implies resistance to all drugs in this class, including methicillin.

By what genetic process can Staphylococcus aureus acquire vancomycin resistance from Enterococcus faecalis?

S. aureus acquires resistance through horizontal gene transfer via conjugation, where a plasmid carrying the \text{vanA} gene is transferred from Enterococcus faecalis.

Which biochemical resistance mechanism is utilized by Staphylococcus aureus strains carrying the vanA gene?

The strain utilizes an altered target site mechanism, modifying the cell wall structure so vancomycin can no longer bind and prevent cell wall synthesis.