Extended-spectrum β-lactamase(s) Gene Transfer of Klebsiella pneumoniae in Raw Foods

Introduction: Raw foods such as vegetables and unpasteurized milk may contain commensal bacteria harboring antibiotic resistance genes, such conditions increase the potential for antibiotic resistant gene transfer. Klebsiella pneumoniae is common commensal bacteria associated with raw agricultural products. Extended-spectrum β-lactamases (ESBLs) gene mainly are encoded on conjugative self-transferable plasmids, contributing to the rapid spread of ESBLs producing K. pneumonia into the community setting.

Purpose: The objective of this work was to investigate the potential spread of ESBL genes in raw foods.

Methods: K. pneumoniae (n = 8) were isolated from 17 different packages of retail seed sprouts. Four of the 8 isolates and KP342 (origin from maize) were used as recipients. ESBL-producing clinical (ATCC 700603) or bovine K. pneumoniae were used as donor isolates. Based on their profile of virulence genotype and antibiotic resistance phenotype, a donor and recipient were mated in TSB, unpasteurized milk (UPM), pasteurized milk (PM), and alfalfa sprouts (AS) at defined temperatures. Transconjugants were verified by amplification of isolate specific virulence genes and transferred ESBL genes by PCR.

Results: All isolates from seed sprouts were resistant to ampicillin. However, none of those isolates were ESBL positive.  Gene transfer in TSB, UPM, and PM occurred at 24°C (room temperature) and 37°C, but not at 4°C. The bla_SHV18 gene of ATCC 700603 was transferred at a frequency of 6.4E-07 to 4.0E-04 in those mating. The bla_CTX-M gene of bovine isolates was transferred to KP342 at a frequency of 6.4E-07 to 3.23E-05 depending on the medium. The transfer rate in alfalfa sprouts was significantly higher than in TSB, UPM, and RM at 24°C (P < 0.05). However, there was no significant difference between 24°C and 37°C.

Significance: This work shows evidence that raw foods which carry high numbers of commensal bacteria can serve as a matrix for ESBLs gene transfer from K. pneumonia.