Antibiotic-resistant Gram-negative Bacteria in Urban Flies, and the Increased Risk Posed by Open-air Markets in Mexico City

Aims: Flies are known to spread antibiotic resistant bacteria (ARB), especially from farms to cities; but they may also play a role in the intra-urban dispersion of ARB, in conjunction with poor sanitary conditions. Here, we characterized gram-negative ARB isolated from urban flies (Lucilia and Sarcophaga spp.), and the co-relation with the periodic installation of two open-air markets in Mexico City.

Methodology: Forty-two flies were individually captured, and 116 gram-negatives (68 of them Escherichia coli) were isolated from them. Resistance prevalence, and the presence of class 1 integrons was assessed.

Results: The isolates were resistant to an average of 2.26 antibiotics (2.6 for E. coli), and 33% of E. coli isolates carried the intI1 gene. Thirteen percent of E. coli isolates produced extended-spectrum beta-lactamases (ESBL), all of them CTX-M, alone or, mostly, along TEM enzymes. Comparing data from market-free days vs. days when open-air markets were installed, the average number of resistance phenotypes per E. coli isolate went from 2.14 to 3.09; the number of resistance phenotypes per fly from 4.62 to 8.88; the average number of resistances per isolate per fly from 1.25 to 2.43; and the ESBL-producing carriage rate per fly from 0.08 to 0.38, respectively (P <.05). Other resistance parameters, were consistently higher among flies captured on market days, but differences were not significant.

Conclusion: Urban flies in Mexico City carry a high number of gram-negative ARB; the presence of open-air markets significantly increase the risk of fly-mediated ARB spreading to the neighboring areas.