Pseudomonas aeruginosa exposed to silver sulfadiazine (AgSu) were examined in an electron microscope. The treated cells were distorted in shape, and structures (blebs) protruded from the cell surface. These "blebs" appeared to arise from the cell wall. A strain of P. aeruginosa resistant to AgSu did not display these changes. Upon exposure of P. aeruginosa to silver nitrate, none of these changes was seen; rather, such cells are characterized by large, central aggregations of nuclear material. The results are consistent with previous findings which suggested that AgSu acted at the cell surface.Although complexes between silver sulfadiazine (AgSu) and deoxyribonucleic acid (DNA) can be formed in vitro (12), there is no evidence that such complexes result in bacteria treated with AgSu. Rather, the biochemical and metabolic evidence suggests (10) that the drug acts at the external cell structures. The present report deals with an electron microscopy study of the effects of AgSu on Pseudomonas aeruginosa. The effects of silver nitrate were also examined. The results appear to confirm the conclusions based on metabolic findings.

Two silver sulfadiazine-resistant isolates of Enterobacter cloacae obtained in a burns unit where the drug was in use were studied. These strains were resistant to elevated levels of the drug, and they were cross-resistant to silver benzoate, but not to silver nitrate. Growth of the strains in nutritionally poor defined media sensitized them to the inhibitory action of the drug. Exposure of the bacteria to penicillins rendered them susceptible to silver sulfadiazine. The resistant bacteria harbored episomes for resistance to carbenicillin and kanamycin; however, resistance to silver sulfadiazine could not be transferred by these episomes. Twenty-three strains of E. cloacae isolated in a general hospital were sensitive to much lower levels of the drug (≤50 μg/ml).

Pasteurella multocida serotype A was found in association with the mucosal epithelium of the nasopharynges of rabbits with respiratory tract infections. The bacteria specifically attached to squamous epithelial cells of the pharyngeal mucosa both in vivo and in vitro and to some tissue culture cell lines such as HeLa. All strains with serotype A capsules were adhesive. With the exception of one serotype D strain, strains with capsular serotypes B, D, and E were at least 10-fold less adhesive. Bacterial adhesiveness was much reduced after pronase digestion, heat treatment, and homogenization, but removal of the hyaluronic acid capsule increased adhesion. Electron microscopy revealed that fimbriae were produced by an adhesive pasteurella strain, but not by two nonadherent strains. The attachment of the former strain to pharyngeal and HeLa cells was inhibited by N-acetyl-D-glucosamine. Together, these findings suggest that this amino sugar may be a component of the receptor on both animal cell surfaces and that the fimbriae may be the adhesions. It is proposed that bacterial attachment has a role in colonization and infection of rabbit upper respiratory mucosae.

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