Image analysis of colony size for investigating sublethal injury in Listeria monocytogenes

The traditional method for measuring sublethal injury in bacteria involves differential plating of injured cells on selective and nonselective media. Image analysis of the size of colonies from injured Listeria monocytogenes cells plated on nonselective media only was investigated as an alternative. With colonies from healthy cells, colony area was normally distributed, but heat- and starvation-stressed cells produced colonies with areas that showed a significant (p less than or equal to 0.05) skewness to the right. Although no relationship between sublethal injury and skewness was apparent, mean colony area was linearly (r(2) = 0.90) related to sublethal injury. Mean colony area can therefore be used as a measure of sublethal injury in Listeria monocytogenes, eliminating the need for differential plating in certain experiments examining this phenomenon. Since significantly different (P less than or equal to 0.05) skewness was apparent between colonies derived from heat-stressed cells as opposed to those derived from starvation-stressed cells, this method also provides further information on variations in injury between individual cells within stressed populations. Due to variations in colony size between strains, the method is limited in many cases to application in single-culture studies.