Enumeration, Resuscitation, and Infectivity of the Sublethally Injured Erwinia Cells Induced by Mild Acid Treatment

Abstract

Current knowledge about microbial injury was derived mostly from studies with bacteria associated with processed foods. Demonstration of injury and repair in phytopathogenic bacteria and its implication on pathogen detection and disease ecology have not been reported. The objective of this study was to investigate the lethal and sublethal effects of acetic acid (AA) on soft-rot Erwinia spp. and the limitation of using selective media for isolation of injured cells. Following exposure to 0.3% AA for 6 min, 90 to 99% of E. carotovora subsp. carotovora, E. carotovora subsp. atroseptica, and E. chrysanthemi cells were killed. When AA-treated samples were plated on agar media, the number of bacteria recovered on nonselective media such as brain heart infusion agar was 3 log units higher than that recovered on selective media such as crystal violet pectate (CVP). Lethal and sublethal actions of AA on Erwinia spp. were affected greatly by acid concentration, exposure time, and bacterial strains tested. Injured Erwinia cells were able to repair and regain the ability to grow on CVP when placed in nutrient broth but not in selective broth containing crystal violet and sodium dodecyl sulfate. Injured cells also were able to resuscitate on cut surfaces of cucumber fruit and to induce soft rot on green bell pepper. Together, these results suggest that direct use of selective media for isolation of Erwinia spp. could limit the recovery of injured cells in samples that have been exposed to chemical or physical stresses. Enrichment of these samples in nutrient broth before plating on CVP is expected to improve the detection of injured Erwinia spp.