American Journal of Clinical Nutrition, Vol 25, 1459-1466, Copyright © 1972 by The American Society for Clinical Nutrition, Inc.

Enteric bacterial interactions in insects

¹ From the Department of Biological Sciences, University of Illinois at Chicago Circle, Chicago, Illinois 60680

The potential of maggots and adult flies to harbor and disseminate Salmonella and Shigella was investigated gnotobiotically after it was found that a majority of larvae and all of the emerging adults breeding in normally contaminated media plus Salmonella typhi or Shigella flexneri failed to retain either pathogen. When flies were raised in the absence of other microorganisms, Shigella flexneri, Salmonella typhi, S. paratyphi B., S. typhimurium, and S. enteritidis, although reduced in numbers, survived pupation. In vivo bicontaminant interactions of S. typhimurium with E. coli, Streptococcus faecalis, and Proteus mirabilis identified P. mirabilis as the major antagonist of Salmonella. Although equal numbers of both organisms were introduced initially, Proteus: Salmonella ratios reached 11,500:1 in the maggot gut, compared with a maximum of 20:1 in a nutrient broth culture. In searching for an in vitro model that would parallel the in vivo antibacterial effects, a filtrate of a 15-day broth culture of Proteus mirabilis at pH 2.5 to 3.0 was found to demonstrate the extremely rapid kill and acid-dependent activity occurring in the gut of the maggot.

In the gnotobiotic adult house fly, inputs of 22 S. typhimurium produced excretion of the pathogen in 27 out of 45 flies, with outputs as high as 1.4 x 10⁷. As input increased, the percentage of Salmonella excreters generally also increased until all flies showed bacteria in their defecations at inputs of 1.3 x 10⁴. The green bottle fly appeared to be a poorer host for Salmonella, with lower percentages of bacterial excreters and multipliers observed. Antagonisms of Salmonella by Proteus were again evident in the adult. The excretion of Salmonella was reduced to 1 day, and the percentage of flies showing excretion on that day was 27%, compared with 8 days of excretion and 87% on the 1st day in the case of monocontaminated flies. A mouse fecal flora substituted for Proteus exerted a lesser degree of antagonism to Salmonella.

Overall data show the adult fly has the most potential for disease transmission, as the maggot has limited motility, and possesses autosterilization mechanisms which limit its capacity to carry pathogens over into the adult stage.

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