Adaptive resistance to aminoglycoside antibiotics in Pseudomonas aeruginosa

Abstract

Summary. Aminoglycoside-resistant variants of Pseudornonas aeruginosa strain P A 0 1 were readily selected by culturing the organism in medium containing increasing concentrations of gentamicin, tobramycin or amikacin until the strains were growing in a concentration of drug 128-fold greater than the minimal inhibitory concentration for the sensitive parent strain. These resistant strains exhibited characteristics previously associated with the impermeability type of resistance mechanism, i.e., they grew more slowly than the parent strain, the resistance was unstable in the absence of the antibiotic, and adaptation to one of the antibiotics conferred crossresistance to other aminoglycosides. The adapted strains grew, with minimal morphological alterations, in concentrations of the various aminoglycosides that normally produced cell envelope damage, misshapen and filamentous cell formation, and cell lysis in the sensitive strain. Neither protein H1 nor phospholipid alterations appear to play a significant role in adaptive resistance to aminoglycoside antibiotics in this model system. The acquisition of adaptive resistance to the aminoglycoside antibiotics did not confer resistance to polymyxin B, another cationic antibiotic which is thought to share binding sites within the outer membrane with the aminoglycosides.

Morphological and ultrastructural changes in bacterial cells as an indicator of antibacterial mechanism of action

“…Antibacterial agent-induced bleb formation has been reported in numerous Gram-negative species including E. coli [41,143,179,180,182], L. pneumophila [83,171], P. aeruginosa [142,[181][182][183], and Salmonella Typhimurium [184]. Unlike bulges, which are typically observed as a single protrusion per cell (Fig.…”

“…With the aminoglycoside gentamicin, blebbing is induced over a wide range of concentrations (1xMIC to 100xMIC [142,181]), sometimes appearing as early as 1 min following treatment [181]. The blebs remain observable until the bacteria lyse, this occurring after around 4 h following high concentration gentamicin treatments [142].…”

“…Examples of RNA and protein synthesis inhibitors that induce blebbing include bicyclomycin [143], arbekacin [179], gentamicin [142,[181][182][183], and erythromycin [83]. With the aminoglycoside gentamicin, blebbing is induced over a wide range of concentrations (1xMIC to 100xMIC [142,181]), sometimes appearing as early as 1 min following treatment [181].…”

“…Inhibition of protein synthesis [127,[140][141][142] (or the RNA synthesis that precedes it [143]) can also induce filamentation, but from the available evidence this increase in cell length is much less than that caused by inhibitors of peptidoglycan or DNA synthesis [52,61,144]. For example, cells of E. coli treated with bactericidal levels of kanamycin undergo just a 1.6-fold increase in size after 3 hours, whereas cells treated with equivalent concentrations of ampicillin or norfloxacin undergo a 9.4-fold increase in the same timeframe [144].…”

“…Bleb formation occurs following treatment with membrane-active agents [24,41,181,184] and inhibitors of RNA [143] and protein synthesis [83,179,181]. All three types of agent also induce filamentation [39, 142,143], and this can accompany the bleb formation [142,143]. Terms used synonymously with 'blebbing' include 'blistering' [24], 'bubbling' [42], and 'swelling' [24].…”