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Applied and Environmental Microbiology, October 1998, p. 3882-3886, Vol. 64, No. 10
0099-2240/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

Acid-Sensitive Enteric Pathogens Are Protected from Killing under Extremely Acidic Conditions of pH 2.5 when They Are Inoculated onto Certain Solid Food Sources

Scott R. Waterman1 and P. L. C. Small2,*

Department of Infectious Diseases, Imperial College, Hammersmith Hospital, London W12 ONN, United Kingdom,1 and Rocky Mountain Laboratories, National Institute of Allergy and Infectious Diseases, Hamilton, Montana 598402

Received 2 January 1998/Accepted 6 July 1998

Gastric acidity is recognized as the first line of defense against food-borne pathogens, and the ability of pathogens to resist this pH corresponds to their oral infective dose (ID). Naturally occurring and genetically engineered acid-sensitive enteric pathogens were examined for their ability to survive under acidic conditions of pH 2.5 for 2 h at 37°C when inoculated onto ground beef. Each of the strains displayed significantly high survival rates under these normally lethal conditions. The acid-sensitive pathogens Campylobacter jejuni and Vibrio cholerae, which were protected at lower levels from acid-induced killing by ground beef under these conditions, were sensitive to killing in acidified media at pH 5.0 but survived at pH 6.0. Salmonella inoculated onto the surface of preacidified ground beef could not survive if the pH on the surface of the beef was 2.61 or lower but was viable if the surface pH was 3.27. This implies that the pH of the microenvironment occupied by the bacteria on the surface of the food source is critical for their survival. Salmonella was also shown to be protected from killing when inoculated onto boiled egg white, a food source high in protein and low in fat. These results may explain why Salmonella species have a higher oral ID of approximately 105 cells when administered under defined conditions but have been observed to cause disease at doses as low as 50 to 100 organisms when consumed as part of a contaminated food source. They may also help explain why some pathogens are associated primarily with food-borne modes of transmission rather than fecal-oral transmission.

* Corresponding author. Mailing address: Rocky Mountain Laboratories, Microscopy Branch, 903 South 4th St., Hamilton, MT 59840. Phone: (406) 363-9280. Fax: (406) 363-9371. E-mail: pam_small{at}nih.gov.

Applied and Environmental Microbiology, October 1998, p. 3882-3886, Vol. 64, No. 10
0099-2240/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.


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