Antimicrobial susceptibility pattern of Helicobacter suis isolates from pigs and macaques

Helicobacter suis is a fastidious, Gram negative bacterium that colonizes the stomach of pigs and non-human primates. It has also been associated with gastric disease in humans. A combined agar and broth dilution method was used to analyze the activity of 15 antimicrobial agents against 20 and 15 H. suis isolates obtained from pigs and macaques, respectively. After 48 h microaerobic incubation, minimal inhibitory concentrations (MICs) were determined by software-assisted calculation of bacterial growth as determined by quantitative real-time PCR. A monomodal distribution of MICs was seen for beta-lactam antibiotics, macrolides, gentamicin, neomycin, doxycycline, metronidazole, and rifampicin. Presence of a bimodal distribution of MICs indicated that 2 porcine isolates did not belong to the wild type population (WTP) for fluoroquinolones. This was also the case for 1 porcine isolate for tetracycline, 1 porcine and 2 primate isolates for lincomycin, and 1 primate isolate for spectinomycin. Single nucleotide polymorphisms (SNPs) were present in the gyrA gene of the isolates not belonging to the WTP for fluoroquinolones and in ribosomal protein encoding genes of the isolates not belonging to the WTP for tetracycline and spectinomycin. MICs of ampicillin, tetracycline and doxycycline were higher for porcine H. suis isolates compared to primate isolates and in these porcine isolates SNPs were detected in genes encoding penicillin binding and ribosomal proteins. This study indicates that acquired resistance occasionally occurs in H. suis isolates and that zoonotically important porcine isolates may be intrinsically less susceptible to beta-lactam antibiotics and tetracyclines than primate isolates.