The increased microbial resistance that frequently follows exposure to an antibiotic has generally been considered to be characteristic of the antibiotic itself and not to entail increased resistance to other antibiotics. This viewpoint has been challenged recently by a number of investigators, whose studies have yielded information that may prove of considerable clinical importance and experimental interest.
Preliminary observations1 indicated that acquired resistance to terramycin was accompanied by decreased sensitivity to aureomycin and that acquired resistance to aureomycin or to chloramphenicol was accompanied by cross resistance to the other agent and to terramycin. Since then, several extensive studies on cross resistance among antibiotics have been reported. Kaipainen2 from the department of serology and bacteriology of the University of Helsinki cultured 27 different bacterial strains in increasing concentrations of aureomycin, chloramphenicol, terramycin, dihydrostreptomycin, and penicillin. After the resistance of the bacteria to the primary antibiotic was increased, simultaneous changes