47-2

A. YOUSEF, Ohio State University, Columbus, OH 43210

Sensitivity of microorganisms to ozone depends largely on the medium, the method of ozone application, and the species. When treated with ozone under similar conditions and in low-ozone-demand media, bacteria vary in sensitivity depending on their cell wall structure, physiological state, and their presence in a vegetative or a spore form.

Bacterial cell surfaces appear to be the primary targets of ozone activity. Double bonds of unsaturated lipids in the cell envelope are probably the primary sites of attack. Lipoprotein and lipopolysaccharide layers of Gram-negative bacteria would be subjected first to attack by ozone, which results in a change in cell permeability, eventually leading to lysis.

Interaction of ozone with microorganisms was elucidated in our laboratory by detecting cell injury and examining treated cells with a scanning electron microscope. When Escherichia coli O157:H7, Pseudomonas fluorescens, Leuconostoc mesenteroides, and Listeria monocytogenes were treated with ozone at 0.55-1.85 mg/L, 51-78% of survivors were injured. The injured population was greater at intermediate rather than high C*t (mg/L x sec) values.

Among the tested microorganisms, P. fluorescens was the most, while L. monocytogenes was the least injured by ozone treatments. Resistance of tested bacteria to ozone followed this descending order: E. coli O157:H7, P. fluorescens, Leu. mesenteroides, and L. monocytogenes.

The electron microscopic analysis showed that ozone caused damage to the cellular structure; this damage was more pronounced in Gram-negative than Gram-positive bacteria. Damage to the spore coat of Bacillus sp. also was observed at low ozone concentrations. When treated under similar conditions, Gram-positive bacteria seemed to lose some mucoid material outside the cell wall, whereas Gram-negative cells tended to collapse and lose cellular components.