Laboratory Diagnosis of Bacterial Infections (II)

Study Notes

Selective media contain specific substances to encourage the growth of target bacteria while inhibiting the growth of other organisms
Thayer Martin media inhibits antibacterial and antifungal agents, and is used to grow pathogenic species of Neisseria.
Löwenstein-Jensen media uses malachite green dye to isolate species of Mycobacterium.
Mannitol salt agar contains high salt content, promoting the growth of Staphylococcus aureus.
MacConkey agar employs bile salts to specifically cultivate Enterobacteriaceae.

Differential media typically contain indicators that alter color based on the metabolic activities of the targeted microorganisms.
MacConkey agar distinguishes between lactose fermenters and non-lactose fermenters.
Mannitol salt agar differentiates based on Staphylococcus aureus's production of acid from mannitol, shown through color changes.

Bacterial colony characteristics (size, texture, color, hemolysis, smell) aid in species identification.
Colonies can be pigmented due to various pigments, either endopigments (e.g., Staphylococcus aureus produces golden yellow endo-pigment) or exopigments (e.g., Pseudomonas aeruginosa produces bluish-green exopigment).
Biochemical tests (catalase, coagulase, and urease tests) are additional tools in determining bacterial species.

Antibiotics sensitivity tests are conducted to ascertain the efficacy of antibiotics against identified bacteria, aiding in selecting the optimal antibiotic treatment.
Common methods include the Kirby-Bauer method (disc-diffusion), E-test (antibiotic diffusion), and broth dilution method.
In Kirby-Bauer testing, the zone of growth inhibition surrounding antibiotic discs is measured, indicating antibiotic susceptibility.
The E-test determines the minimum inhibitory concentration (MIC).
The broth dilution method involves serial dilutions of antibiotics and measures the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC). The MIC is the lowest antibiotic concentration to stop observable bacterial growth. MBC is the lowest concentration to eliminate the bacteria.

Antigen detection in specimens quickly identifies bacterial components, making it rapid and simple to perform.
Identifying bacterial antigens in samples like throat swabs, stool samples, or cerebral spinal fluid (CSF) are common antigen detection methods.
Culture methods are still needed to confirm bacterial identity and antibiotic sensitivity determination.

DNA is unique, enabling bacterial identification based on DNA detection
Polymerase Chain Reaction (PCR) is a common molecular method.
PCR's advantages include: rapid, sensitive, and specific identification of bacteria.
PCR involves three main steps: denaturation, annealing, and extension.