Estimation of the state of the bacterial cell wall by fluorescent In situ hybridization

Fluorescent in situ hybridization (FISH) is now a widely used method for identification of bacteria at the single-cell level. With gram-positive bacteria, the thick peptidoglycan layer of a cell wall presents a barrier for entry of horseradish peroxidase (HRP)-labeled probes. Therefore, such probes do not give any signal in FISH unless cells are first treated with enzymes which hydrolyze the peptidoglycan. We explored this feature of FISH to detect cells which have undergone permeabilization due to expression of autolytic enzymes. Our results indicate that FISH performed with HRP-labeled probes provides a sensitive method to estimate the states of cell walls of individual gram-positive bacteria.