Response of Listeria monocytogenes to Disinfection Stress at the Single-Cell and Population Levels as Monitored by Intracellular pH measurements and viable-cell counts.
Listeria monocytogenes has a remarkable ability to survive and persist in food production environments. The purpose of the present study was to determine if cells in a population of L. monocytogenes differ in sensitivity to disinfection agents as this could be a factor explaining persistence of the bacterium. In situ analyses of Listeria monocytogenes single cells were performed during exposure to different concentrations of the disinfectant Incimaxx DES to study a possible population subdivision. Bacterial survival was quantified with plate counting and disinfection stress at the single-cell level by measuring intracellular pH (pH(i)) over time by fluorescence ratio imaging microscopy. pH(i) values were initially 7 to 7.5 and decreased in both attached and planktonic L. monocytogenes cells during exposure to sublethal and lethal concentrations of Incimaxx DES. The response of the bacterial population was homogenous; hence, subpopulations were not detected. However, pregrowth with NaCl protected the planktonic bacterial cells during disinfection with Incimaxx (0.0015%) since pH(i) was higher (6 to 6.5) for the bacterial population pregrown with NaCl than for cells grown without NaCl (pH(i) 5 to 5.5) (P < 0.05). The protective effect of NaCl was reflected by viable-cell counts at a higher concentration of Incimaxx (0.0031%), where the salt-grown population survived better than the population grown without NaCl (P < 0.05). NaCl protected attached cells through drying but not during disinfection. This study indicates that a population of L. monocytogenes cells, whether planktonic or attached, is homogenous with respect to sensitivity to an acidic disinfectant studied on the single-cell level. Hence a major subpopulation more tolerant to disinfectants, and hence more persistent, does not appear to be present.
Kastbjerg
Vicky G
VG
National Institute of Aquatic Resources, Technical University of Denmark, Kongens Lyngby, Denmark. vgk@aqua.dtu.dk
Nielsen
Dennis S
DS
Arneborg
Nils
N
Gram
Lone
L
D015169:Colony Count, Microbial; D003593:Cytoplasm / Q000737:chemistry*; D004202:Disinfectants / Q000494:pharmacology*; D006863:Hydrogen-Ion Concentration; D008089:Listeria monocytogenes / Q000187:drug effects*; D050296:Microbial Viability / Q000187:drug effects*; D008856:Microscopy, Fluorescence / Q000379:methods; D012965:Sodium Chloride / Q000378:metabolism; D013312:Stress, Physiological
D004202:Disinfectants; D012965:Sodium Chloride