Bacitracin Susceptibility Test- Principle, Procedure, Results, Uses


Bacitracin susceptibility test is a simple and rapid method to differentiate group A streptococci (GAS) from other beta-hemolytic streptococci (BHS) based on their sensitivity to a low concentration of bacitracin, an antibiotic produced by Bacillus subtilis. It is also used to distinguish micrococci from staphylococci based on their susceptibility to bacitracin.

Bacitracin is a polypeptide antibiotic that inhibits the synthesis of bacterial cell wall by interfering with the transport of peptidoglycan precursors across the cytoplasmic membrane. GAS are highly susceptible to bacitracin and show a clear zone of inhibition around a bacitracin disk, whereas other BHS are resistant and show no zone of inhibition. Micrococci are also susceptible to bacitracin, while staphylococci are resistant.

Bacitracin susceptibility test is performed by placing a filter paper disk impregnated with 0.04 units of bacitracin on a blood agar plate inoculated with the test organism. After incubation, the presence or absence of a zone of inhibition around the disk is observed and measured. A zone of inhibition of 10 mm or more indicates susceptibility, while no zone or a zone of less than 10 mm indicates resistance.

Bacitracin susceptibility test is useful for the presumptive identification of GAS, which are the most common cause of pharyngitis, impetigo, cellulitis, necrotizing fasciitis, and rheumatic fever. It can also help in the identification of micrococci, which are opportunistic pathogens that can cause endocarditis, septicemia, and wound infections.

Bacitracin susceptibility test is not a definitive test and should be confirmed by other methods such as serological grouping or molecular typing. It also has some limitations such as false-positive results due to light inoculum or old media, false-negative results due to different concentrations of bacitracin or different media, and variable results due to some strains of GAS, BHS, micrococci, and staphylococci that show intermediate susceptibility to bacitracin.