Effect of antibiotics on L-form induction of Neisseria meningitidis

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

Efforts to reduce the global burden of bacterial disease and contend with escalating bacterial resistance are spurring innovation in antibacterial drug and biocide development and related technologies such as photodynamic therapy and photochemical disinfection. Elucidation of the mechanism of action of these new agents and processes can greatly facilitate their development, but it is a complex endeavour. One strategy that has been popular for many years, and which is garnering increasing interest due to recent technological advances in microscopy and a deeper understanding of the molecular events involved, is the examination of treated bacteria for changes to their morphology and ultrastructure. In this review, we take a critical look at this approach. Variables affecting antibacterial-induced alterations are discussed first. These include characteristics of the test organism (e.g. cell wall structure) and incubation conditions (e.g. growth medium osmolarity). The main body of the review then describes the different alterations that can occur. Micrographs depicting these alterations are presented, together with information on agents that induce the change, and the sequence of molecular events that lead to the change. We close by highlighting those morphological and ultrastructural changes which are consistently induced by agents sharing the same mechanism (e.g. spheroplast formation by peptidoglycan synthesis inhibitors) and explaining how changes that are induced by multiple antibacterial classes (e.g. filamentation by DNA synthesis inhibitors, FtsZ disruptors, and other types of agent) can still yield useful mechanistic information. Lastly, recommendations are made regarding future study design and execution.

Endotoxin release from neisseria meningitidis. Relationship between key bacterial characteristics and meningococcal disease

A method was established in order to measure total and filtrable (CF) endotoxin in cultures of Neisseria meningitidis strains. The Limulus lysate (LAL) test gave results which paralleled those of the standard rabbit pyrogen test and of gas chromatography (GLC), concerning detection of different CF endotoxin levels. Meningococci varied in their ability to produce CF endotoxin. Cultures of similar bacterial densities, whether with a high (E+) or a low (E-) release of endotoxin in CF, had both high yields of endotoxin in sonicated culture suspensions determined by the LAL test. GLC demonstrated only small differences in total LPS contents between E+ and E- strains. This suggests that strains with similar cell wall endotoxin contents may vary in CF endotoxin. Electron microscopy revealed that E+ strains presented a high number of free, outer cell wall fragments (blebs, tubuli, membranes and aggregates of such structures) in surroundings. Few such free, small structures were found around E- strains. The amount of CF endotoxin of E+ strains was in part a function of the number of colony forming units (CFU/ml), and generally followed the growth curve. Because of its moment of appearance, and also based on electron microscopy findings, CF endotoxin appeared mainly to be released from living bacteria. The CF level of endotoxin was low or not detected at all in cultures of E- strains although their cultures reached higher mean CFU-levels than the niveaus required for the detection of CF endotoxin in the E+ strains. The E+ property was strain dependent. Meningococci isolated from CSF or blood had a significantly higher proportion of E+ strains (88.2%) and a higher CF endotoxin titre (greater than or equal to 10(3); 34.5%), than isolates from carriers (32.3% and 10.8%, respectively) (p less than 0.001 and p less than 0.001, respectively). A high mean CFU/ml in cultures seemed to be more often associated with isolates from patients than from carriers, more often with the presence than abscence of capsular polysaccharide (p less than 0.05), and more often with the presence than absence of the E+ property (p = 0.002). E+ strains were mostly serogroupable (i.e. encapsulated), regardless of source of the isolate (99% case and 80% carrier isolates). In contrast, serogroupable bacteria were not necessarily E+ when isolated from carriers (54.8%). The serogroup most apt to cause disease tended to have the highest proportion of E+ strains and the highest level of CF endotoxin.(ABSTRACT TRUNCATED AT 400 WORDS)

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

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.

Events leading to cell death and lysis of Neisseria meningitidis in low concentrations of penicillin G

Neisseria meningitidis SD1C exhibited a low tolerance to penicillin G (0.03 microgram/ml). Loss of viability in the absence of polyvinylpyrrolidone-40 and horse serum was independent of the concentration of antibiotic above the minimum inhibitory concentration, whereas the rate of bacteriolysis was concentration dependent. Penicillin-induced lysis was a secondary event in this organism. At low levels of penicillin G, growth characteristics, i.e., absorbancy changes, respiratory rate, and uptake of Mg2+, appeared normal during the first 90 min in penicillin; however, viability dropped dramatically. Additionally, total cell numbers remained constant while cell mass continued to increase at a rate normal for the population. The increase in cellular mass in the absence of cell division could be observed microscopically. Only one ultrastructural change induced by penicillin correlated with the loss in viability: the loss in continuity of the outer membrane with the peptidoglycan but only at the site of septum formation. This lesion did not occur when cells were grown in media supplemented with the protective agents polyvinylpyrrolidone-40 and horse serum. Under these conditions of growth and with relatively high levels of penicillin, constant viability was maintained, but cell division no longer occurred. Cell populations treated with penicillin in the presence of the protective agents became increasingly more dependent on the presence of these agents for total viability even in the absence of penicillin in the culture.

Electron microscopical study of Neisseria meningitidis releasing various amounts of free endotoxin

A study has been made of the ultrastructure of four strains of Neisseria meningitidis which liberate varying amounts of free endotoxin in a chemically-defined, protein-free medium. The two strains which did not release detectable or only sparse amounts of free, filtrable endotoxin were rather uniform in cell size. Their cells appeared to be intact and showed a low tendency to aggregate, In addition cells of these strains showed only sporadic loose, trilaminar membranes and blebs, and free membranous structures were sparse in the medium. The endotoxin releasing strains liberated a high yield of free structures from the outer cell wall into the medium. These structures may represent the lipopolysaccharide (LPS).

Mortality in meningococcal infections

Among 124 patients with meningococcal infections between 1966-1976, 108 had meningitis with a fatality rate of 7.4%, and 16 septicaemia with 18.8% fatality. High risk patients (more than 3 prognostically unfavourable characteristics) increased from 3.5% 1966-1970 to 15.6% 1971-1976, and the total mortality from 3.6 to 10.4%. The most unfavourable prognostic factor was low blood pressure at admission. High risk patients were more common among those who had meningococci with complete or partial sulfonamide resistance (25.7%) than among those having sensitive meningococci (11.9%). When the meningococci were at least partially sulfonamide-resistant, initial therapy with the combination of benzylpenicillin, sulfaisodimidine and chloramphenicol seemed to be more successful than benzylpenicillin alone in patients above 10 years. Ampicillin was used only in children, and these all survived.