Growth kinetics of Campylobacter and ESBLs in selective enrichment broths

According to the ISO-protocol for detection of thermotolerant Campylobacter spp. in food and animal feeding stuffs (ISO 10272-1, 2006), Bolton broth (BB) is used which is mixed 10:1 with the food sample. After enrichment, campylobacters are isolated on mCCDA, which is often complicated, since abundantly growing Extended Spectrum Beta-Lactamase producing bacteria (ESBLs) make it hard to recognize and isolate Campylobacter colonies. Other enrichment broths, such as Preston broth (PB) and BB plus clavulanic acid (BBc) have been suggested. However, detailed growth dynamics of Campylobacter and its competitors during enrichment remain unclear, where these would provide a solid base for further improvement of the enrichment procedure of Campylobacter. With a revision of the ISO-10272-protocol at hand, we measured the growth kinetics in PB, BB and BBc of several strains of C. jejuni and ESBLs separately and in combination. Also, growth dynamics were evaluated during enrichment of naturally contaminated chicken samples. No significant difference in growth kinetics were found using a pre-enrichment step of 4 h at 37°C compared to immediate enrichment at 41.5°C. The yields and often the growth rates of Campylobacter in co-culture with ESBLs were lower than in pure cultures, indicating severe suppression of Campylobacter by ESBLs. PB and BBc successfully inhibited growth of ESBLs and are therefore a better choice as enrichment medium for possibly ESBL-contaminated samples. Therefore, the choice in the revised ISO-10272-protocol for use of PB in samples where high levels of background flora is suspected, is well supported by these experiments.