Non-typhoidal Salmonella contamination in egg shells and contents from retail in Western Australia: Serovar diversity, multilocus sequence types, and phenotypic and genomic characterizations of antimicrobial resistance

In recent years, the number of human salmonellosis cases in Western Australia (WA) has increased more dramatically than in any other Australian state. In 2017, the number of cases in WA was more than double the five-year average, and eggs had emerged as the key culprit for several Salmonella foodborne disease outbreaks. To better understand such an epidemiologically intriguing situation, our research goal was to investigate the prevalence, serovar diversity, multilocus sequence types, and antimicrobial resistance of non-typhoidal Salmonella contamination in retail eggs produced and sold in WA. A total of 200 visually clean and intact retail egg samples (each containing a dozen eggs) were purchased for one year (2017–2018) from supermarkets in metropolitan Perth, the capital of WA. For each sample, the contents and shells of the 12 eggs were separately pooled and cultured according to standard methods. Overall, Salmonella was detected in 11.5% (23/200) of the tested egg samples. Salmonella was isolated from 4.5% (9/200) and 3% (6/200) of eggshells and egg contents, respectively. In 4% (8/200) of the samples, Salmonella was recovered from both eggshell and egg contents. Isolates from positive retail egg samples were serotyped as either S. Typhimurium (52.2% [12/23]) or S. Infantis (39.1% [9/23]). Both serotypes were concurrently recovered from two different retail egg samples. We retained a set of both S. Typhimurium (n = 29) and S. Infantis (n = 12) isolates from all Salmonella-positive retail packs (n = 23) for further characterization. Only two (S. Typhimurium) isolates showed resistance to ampicillin, of which one carried β-lactamase resistance gene bla_TEM-1b. The remaining isolates (39/41) were susceptible to all 14 antimicrobials included in the minimum inhibitory concentrations (MICs) testing panel. Multilocus sequence typing and serotyping were perfectly mirrored, as all S. Typhimurium isolates were characterized as sequence type (ST)-19, and all S. Infantis isolates were ST-32. This study points to the noteworthy Salmonella prevalence rate in retail egg samples in WA. Our results illustrate minimal public health risks arising from antimicrobial resistance Salmonella from Australian eggs.