Aims:To investigate the effect of a therapeutic and sub-therapeutic chlortetracycline treatment on tetracyclineresistant Salmonella enterica serovar Typhimurium DT104 and on the commensal Escherichia coli in pig. Methods and Results: Salmonella Typhimurium DT104 was orally administered in all pigs prior to antibiotic treatment, and monitored with the native E. coli. Higher numbers of S. Typhimurium DT104 were shed from treated pigs than untreated pigs. This lasted up to 6 weeks post-treatment in the high-dose group. In this group, there was a 30% increase in E. coli with a chlortetracycline minimal inhibitory concentration (MIC) > 16 mg l )1 and a 10% increase in E. coli with an MIC > 50 mg l )1 during and 2 weeks post-treatment. This effect was less-pronounced in the low-dose group. PCR identified the predominant tetracycline resistance genes in the E. coli as tetA, tetB and tetC. The concentration of chlortetracycline in the pig faeces was measured by HPLC and levels reached 80 lg g )1 faeces during treatment. Conclusion: Chlortetracycline treatment increases the proportion of resistant enteric bacteria beyond the current withdrawal time. Significance and Impact of the Study: Treated pigs are more likely to enter abattoirs with higher levels of resistant bacteria than untreated pigs promoting the risk of these moving up the food chain and infecting man.

Aim:To assess the effect of the growth promoter avilamycin on emergence and persistence of resistance in enteric bacteria in the pig. Methods and Results: Pigs (treated with avilamycin for 3 months and controls) were challenged with multiresistant Salmonella Typhimurium DT104 and faecal counts were performed for enterococci, Escherichia coli, S. Typhimurium and Campylobacter (before, during and 5 weeks post-treatment). Representative isolates were tested for antibiotic resistance and for the presence of resistance genes. Avilamycin-resistant Enterococci faecalis (speciated by PCR) were isolated from the treated pigs and continued to be detected for the first week after treatment had ceased. The avilamycin-resistance gene was characterized by PCR as the emtA gene and speciation by PCR. MIC profiling confirmed that more than one strain of Ent. faecalis carried this gene. There was no evidence of increased antimicrobial resistance in the E. coli, Salmonella and Campylobacter populations, although there was a higher incidence of tetB positive E. coli in the treated pigs than the controls. Conclusion: Although avilamycin selects for resistance in the native enterococci population of the pig, no resistant isolates were detected beyond 1 week post-treatment. This suggests that resistant isolates were unable to persist once selective pressure was removed and were out-competed by the sensitive microflora. Significance and Impact of the Study: Our data suggest the risk of resistant isolates becoming carcass contaminants and infecting humans could be minimized by introducing a withdrawal period after using avilamycin and prior to slaughter.

This study provides clear evidence that even a short period of dipping of Collatamp implants, and probably other GCCI, before insertion into the patient results in a significant loss of gentamicin which may be of clinical significance unless the period of soaking is very short. We therefore recommend that wetting of these implants before insertion is not undertaken.

The fluoroquinolones produce multiple photodegradation products. Little is known about these products, particularly whether any possess antimicrobial activity. To investigate this, we used the parallel-line bioassay to investigate discrepancies in zone of inhibition size in conjunction with high performance liquid chromatography (HPLC) analysis. A continuous flow photochemical reaction unit ('Beam-Boost') was used to partially photodegrade the fluoroquinolones ofloxacin, levofloxacin, ciprofloxacin and moxifloxacin (0.02 mM) by between 15 and 89%, as confirmed by HPLC. The concentration of residual parent fluoroquinolone in each irradiated sample was measured by HPLC and a non-irradiated control solution was prepared at the same concentration. These were compared by parallel-line bioassays using Escherichia coli, Enterobacter cloacae and Klebsiella oxytoca. With ofloxacin and levofloxacin, the zone size for the control solution was significantly less than that of the irradiated solutions, with >15% photodegradation in at least two of the indicator organisms, indicating that the photodegradation products possess antimicrobial activity. No difference was seen with ciprofloxacin at any level of photodegradation with any of the indicator organisms, nor with moxifloxacin at 30 and 54% photodegradation. A significant difference was observed with E. cloacae only, at 83% photodegradation.