Change of host range in a resistance factor

Abstract

SUMMARYAn R factor for ampicillin and streptomycin resistance (AS) was identified inSalmonella enteritidis. The AS factor transferred freely toEscherichia coliK12, but only two of 260 K12(AS) clones from this cross would transfer AS toS. typhimurium, although all lines tested transferred it toS. enteritidisand K12. A study of one of the two exceptional lines“ revealed that it also transferred AS toS. paratyphiB,S. thompsonandS. anatum. The R factor maintained its transferability when cycled between these serotypes and K12. Transfer toS. enteritidis, however, resulted in loss of the ability of AS to transfer to the heterologous serotypes, that is, it apparently became host specific forS. enteritidis. S. paratyphiB andS. anatumalso imposed host specificity on AS, butS. typhimuriumandS. thompsondid not. The R factor would always enterS. enteritidis, whatever its previous salmonella host but, once it had done so, it became specific forS. enteritidis. AS could always transfer to K12, which did not seem to modify its host range. These phenomena are most easily explained by analogy with host-controlled modification of phage. Their possible significance in relation to apparent host specificity of R factors is discussed.