Anomalous cellular morphology and growth characteristics of Neisseria meningitidis in subminimal inhibitory concentrations of penicillin G

Abstract

The effects of subminimal inhibitory concentrations of penicillin G on Neisseria meningitidis in the presence and absence of selected stabilizers were examined. Subminimal inhibitory concentrations of penicillin G decreased cell numbers and altered both colonial and ultrastructural morphologies of this meningococcus. Although these levels of penicillin did not have immediate adverse effects on cell mass increase, deoxyribonucleic acid synthesis, or the incorporation of [3H]leucine into protein, they did significantly alter the division rate and the integrity of the cell envelope. The inability of many of the abnormal membranous cell types produced in subminimal inhibitory concentrations of penicillin to form either complete or properly oriented division septa and the overproduction of cell wall material at such sites was indicative of the disruptive effects of this antibiotic on functions necessary for maintaining the normal division process. The addition of the stabilizers polyvinylpyrrolidone-40 and horse serum to the test medium diminished the effects of penicillin G as evidenced by a fourfold increase in the minimal inhibitory concentration. Moreover, such stabilizers maintained the association of the outer membrane with the peptidoglycan and inner membrane.The exact mechanism(s) by which penicillin compounds exert their bactericidal effect is still not understood. The fact that ,8-lactam antibiotics can invoke a wide spectrum of morphological and physiological responses in a variety of bacteria has made this problem more complex.

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

“…These pseudomulticellular bacteria have been proposed to result either from β-lactams inhibiting the peptidoglycan hydrolases required for septal lysis [152,158], or perhaps more likely, from β-lactams altering the cell wall composition and steric configuration in such a way that these enzymes cannot act [152]. This change has been widely reported in bacteria treated with sub-MIC levels (0.06xMIC to 0.5xMIC) of β-lactams [111,125,148,155,158], where it can occur within 3.5 hours [155] and remain observable up to 14 hours following exposure [125]. Prior to [151] or after [84] this window, septal wall thickening is detected but not pseudomulticellular bacteria.…”

“…4), cells that appear either swollen or elongated depending on whether the species divides in multiple planes [125,148] or just one [149,150]. Such bacteria have been variously described as 'large multiseptate organisms' [151], 'pseudomulticellular bacteria' [60,152], and 'multicellular clusters' [153].…”

“…Pseudomulticellular bacteria can also arise following treatment with supra-MIC levels (10xMIC) of β-lactams, but these are more short-lived, cell lysis occurring after 8 hours [150]. Both Gram-positive [150,158] and Gram-negative [125,127,148] cocci can be affected. Pseudomulticellular bacteria with a similar appearance to those induced by β-lactams (ie.…”

“…aureus [150,155] and Enterococcus faecalis [157,188], but can also occur in Gram-negative cocci [125,148] and Gram-positive bacilli [69,189].…”

“…Given their outward appearance as abnormally large or long cells, pseudomulticellular bacteria could potentially be mistaken for spheroplasts or other phenotypes unless transmission electron microscopy (TEM) is performed to detect the multiple septa. Antibacterial drugs known to induce this change include peptidoglycan synthesis inhibitors in the β-lactam [148,150,155] and glycopeptide [153] classes, the protein synthesis inhibitors chloramphenicol [60], lincomycin [125] and minocycline [156], and the folic acid synthesis inhibitor trimethoprim [157].…”