Penicillinsusceptibility

Penicillinsusceptibility is the property of bacteria to be inhibited by penicillin antibiotics. It is assessed in vitro using standardized methods and helps predict the likelihood of clinical success with penicillin therapy.

Susceptibility depends on factors including beta-lactamase production, altered penicillin-binding proteins, and membrane permeability, which determine whether the drug can reach its target in the bacterial cell wall.

Laboratory testing includes disc diffusion, broth microdilution, and E-test to determine minimum inhibitory concentration (MIC). Results are compared with breakpoints from CLSI or EUCAST, classifying isolates as susceptible, intermediate, or resistant.

Clinically, penicillins are typically effective against many streptococci and some enterococci and certain anaerobes when susceptibility is present, and are often paired with beta-lactamase inhibitors or used for susceptible Gram-positive pathogens. Resistance mechanisms such as beta-lactamase production, especially in Gram-negatives, and the presence of altered PBPs (e.g., MRSA PBP2a) limit utility.

Limitations include testing discrepancies, inoculum effects, inducible beta-lactamases, and pharmacokinetic/pharmacodynamic factors that influence in vivo efficacy. Thus susceptibility testing supports, but does not guarantee, clinical success and must be integrated with site of infection, drug exposure, and prevailing resistance patterns.