Exploring the relationship between attachment and antibiotic resistance of Escherichia coli from swine manure

Antibiotics are widely utilized in swine production for treatment and prevention of disease, growth promotion, and to improve the efficiency of feed. Antibiotic resistance has been present since the invention and subsequent use of antibiotics and bacteria have used antibiotic resistant mechanisms to overcome attacks in the environment. Under antibiotic pressure resistant mutants can spontaneously form and resistance genes can be passed between bacteria. Use of antibiotics at subtherapeutic levels is thought to add selective pressure for the development of antibiotic resistant bacteria both enteric and in the environment. However, there is still a large knowledge gap in understanding why some bacteria develop resistance rapidly and others remain susceptible. The ability to attach to surfaces has been shown to result in bacterial persistence in the environment. Presence of attachment factors, such as pili, could be linked to the persistence of uropathogenic E. coli in humans. Few studies have been completed investigating the mechanisms responsible for adhesion of bacteria in the environment, and even fewer still have been performed examining relationships between attachment and antibiotic resistance. Understanding these relationships might have an impact on the timing and methodology of manure application or the use of antibiotics utilized in both human and animal prophylaxis at subtherapuetic levels in feeding operations. The objectives of this study were to: 1) detect and quantify the fraction of bacteria isolated from manure collected from conventional and organic swine production facilities in Iowa showing preferential attachment to quartz; 2) quantify the levels of resistance (susceptible, intermediate, or resistant) of isolates collected from conventional and organic swine production facilities to amoxicillin (AMX), ampicillin (AMP), chloramphenicol (CMP), chlortetracycline (CTC), erythromycin (ERY), gentamycin (GEN), kanamycin (KAN), nalidixic acid (NAL), neomycin (NEO), tetracycline (TET), tylosin (TYL), streptomycin (STP), and sulfamethazine (SMZ); and, 3) statistically quantify any relationships between antibiotic resistance and attachment under different management practices (conventional and organic). E. coli isolates were enumerated from manure collected from six swine production facilities under two management systems—organic, with no antibiotics given and conventional, with antibiotics fed sub-therapeutically. Isolates were subjected to an attachment assay using quartz as a model for fine sand to assess presence of adhesion. A known quantity of each isolate (0.5 McFarland Standard) was added to a conical tube with adequate quartz surface for attachment of all bacteria, after mixing for 20 minutes and settling for five minutes, supernatant was sampled and the bacteria present were enumerated. Attachment was calculated as the difference between the input quantity of bacteria and the output quantity (i.e. the quantity of bacteria in the supernatant). Results show a significantly (p \u3c .0005) higher relationship between conventional swine management and attachment. Antibiotic resistance was quantified using 13 antibiotics at susceptible, intermediate, and resistant MIC concentrations. Results from this study found that E. coli isolated from manure produced under conventional management systems had statistically higher resistance to AMX, AMP, CTC, ERY, KAN, NEO, STP, TET, and TYL; interestingly, a higher level of susceptibility to NAL was found under this management system. A statistically significant relationship was not shown between antibiotic resistance levels and attachment of E. coli from conventional systems, but was for organic management systems. Further understanding of the relationship between antibiotic resistance and attachment under organic and conventional systems is critical to understanding and potentially preventing exposure of human populations to antibiotic-resistant bacteria