Antimicrobial resistance of Salmonella spp. and Escherichia coli isolated from table eggs

The antimicrobial sensitivity of Salmonella spp. and Escherichia coli isolated from the shells and contents of table eggs sampled from sale outlets in Trinidad was determined using the disc diffusion method. The phage types of S. Enteritidis isolates, the phenotypic characteristics of E. coli isolates and the presence of O157 strain were also investigated. Of a total of 74 isolates of Salmonella tested, 17 (22.9%) exhibited resistance to one or more of the seven antimicrobial agents used compared with 104 (88.1%) of 118 E. coli isolates. The difference was statistically significant (P < 0.05; X2). For both microorganisms, resistance was relatively high to streptomycin (54.2%) and tetracycline (35.9%) but low to gentamicin (11.5%) and sulphamethoxazole/trimethoprim (9.4%). Only 1 (1.4%) isolate of Salmonella was multi-resistant while 55 (46.6%) of E. coli isolates were resistant to three or more antimicrobial agents. The frequency of resistance to antimicrobial agents amongst both bacteria was not significantly (P > 0.05; X2) affected by the location of isolation on the egg (shell or content) or source of eggs (farms, shopping malls or other retailers). Eight (19.5%) of 41 S. Enteritidis isolates tested were resistant compared to 4 (26.7%) of 15 isolates of S. Ohio. All S. Enteritidis isolates belonged to phage type 1 (PT1) and all E. coli isolates were non-haemolytic, non-mucoid, sorbitol fermenters and non-O157 strains. It was concluded that the relatively high resistance amongst the bacteria tested could pose therapeutic problems in consumers, particularly in egg-borne salmonellosis or colibacillosis.