sentence: In contrast the occurrence of tetracycline resistance among Swedish poultry isolates has varied dramatically during the same period , despite no use of tetracyclines in poultry production [ 39 ] , illustrating that resistant clones can expand in the absence of selective pressure from antibiotics.The only isolate of Yersinia enterocolitica was positive for the gene encoding the Ail outer membrane protein found in pathogenic Yersinia , and the heat-stable enterotoxin Y-STb , but negative for all other virulence markers investigated including markers for the pYV virulence plasmid and the chromosomal high pathogenicity island .