ß-Lactamases in Salmonella enterica isolated in Australia

Understanding the antibiotic susceptibility of Salmonella enterica is important both from a clinical treatment and a public health perspective. The emergence of extended spectrum ß-lactamases (ESßLs) and AmpC ß-lactamases in S. enterica is important, as this will limit treatment options and could provide a strain with a significant selective advantage. The aim of the study was to screen isolates of S. enterica, including isolates that had previously shown antibiotic resistance, to gauge the extent of ß-lactamase activity in S. enterica in Australia. Phenotypic detection involved screening in accordance with Clinical and Laboratory Standards Institute double disk synergy test guidelines and assessing susceptibility to cefoxitin. Presumptive positives were then screened using a MAST® AmpC and ESßL detection set. S. enterica isolates that were consecutively received in the laboratory (n=624), or had previously exhibited some antibiotic resistance (n=351), were screened for ß-lactamase activity. None of the isolates in the second group were included in the first. ß-lactamase activity was detected in nine of the consecutively received isolates; one with demonstrated ESßL activity and eight others with demonstrated AmpC ß-lactamase. ß-lactamase activity was detected in 16 of the isolates that had previously demonstrated some antibiotic resistance; three with demonstrated ESßL activity and 13 others with demonstrated AmpC ß-lactamase activity. S. enterica serovar Stanley is a serovar that is frequently acquired overseas and this serovar had the highest proportion of isolates that demonstrated ß-Lactamase activity in consecutively sampled isolates (4.95%), reflecting the emergence of an epidemic clone within South East Asia. While antibiotic resistance is being detected in Salmonella isolates, the data indicates that there is limited awareness of, or screening for, ß-lactamases in S. enterica. This study will help to overcome these deficiencies and provide some baseline surveillance data against which future trends can be measured.