Ultrastructural alterations of bacteria during the postantibiotic effect.

Ultrastructural alterations of Staphylococcus aureus and Pseudomonas aeruginosa were examined during the postantibiotic effect (PAE) with transmission electron microscopy. After exposure to dicloxacillin the staphylococci were characterized by an increase in the number of crosswalls, rifampin produced thickening of the cell wall, but only minimal changes were induced by gentamicin. Intracellular electrondense aggregates were observed in P. aeruginosa after exposure to imipenem, tobramycin and ciprofloxacin, and imipenem caused globoid cell formations. These alterations were not uniform in every organism, but they correlated well with the duration of the PAE determined by viable counts.