Recovery of Stressed Microorganisms
Microorganisms present in processed food, water or the environment may be injured and hence more exacting in their growth requirements. Such organisms may be difficult to detect because they fail to grow on the selective media normally used in their isolation. However, under suitable conditions, injured cells can repair cellular damage and recover all their normal properties, including virulence. An appreciation of the nature of sublethal injury and its repair is therefore important in detecting and enumerating microbes. Injured cells may show an extended lag phase, restricted temperature range for growth and increased sensitivity to selective agents, salt, acidity and oxidative stress. In some cases it has been possible to identify the site of cellular injury leading to the observed phenotype. For example, damage to the Gram-negative outer membrane causes increased sensitivity to bile salts and some antibiotics, whilst disruption of cytoplasmic membrane function is probably related to increased sensitivity to salt and acid. Oxidative stress may arise from reactive oxygen species generated metabolically or present in trace amounts in recovery media. The reasons underlying some effects, such as a restricted temperature range for growth, are unknown. In order to recover injured cells it is necessary to allow resuscitation to take place on non-selective solid or in liquid media. In addition to improving the sensitivity of detection methods, resuscitation improves consistency and reduces variability between laboratories.
Recent developments in recovering injured cells have focused on defining the time needed for repair of individual cells within a population and in formulating resuscitation media and/or conditions to avoid exposing cells to oxidative stress. The need to monitor the performance (selectivity and productivity) of