Methods to Detect Stressed Microorganisms 1.

A population of surviving bacteria, after a sublethal physical or chemical treatment, is composed of the stressed or injured and the uninjured or normal cells. Injured cells develop sensitivity to many chemical compounds due to damage to their permeability barriers in the cell wall and the cell membrane. They also fail to repair this damage in the presence of selective compounds and consequently fail to multiply. However, the cells can repair this damage relatively rapidly in a nutritionally adequate environment in the absence of selective agents. Repaired cells regain their resistance to the selective compound and can multiply in a selective environment. Many nonsterile or semipreserved foods are exposed to different kinds of sublethal treatments during processing, handling and storage and thus can contain indicator and pathogenic bacteria in the injured state. These organisms, because of their resistance to many selective compounds, are normally enumerated and isolated with selective media. The injured cells, due to their developed sensitivity will not be detected in these media. However, repaired cells can be detected in these media. Based on these principles two methods, designated as 'liquid-repair' and 'solid-repair' methods, have been developed in our laboratory. The liquid-repair method is effective for enumeration by the MPN technique and isolation of pathogenic and indicator bacteria from different types of semipreserved foods. The solid-repair method in principle can be used for direct enumeration of any organism which is usually enumerated by the selective plating procedure.