Salt-induced L-forms of Staphylococcus aureus

Cell wall deficiency and its effect on methicillin heteroresistance in Staphylococcus aureus

Clinical strains of Staphylococcus aureus with different phenotypic methicillin susceptibility characteristics, bearing or lacking the mecA gene, were tested for their ability to transform into a cell wall-deficient state under special conditions of cultivation. Conversion to L-form growth with formation of typical L-form 'fried egg' colonies and expression of oxacillin resistance was observed in sensitive (mecA-negative) and heteroresistant (mecA-positive) strains. Transmission electron microscopy observation of these strains revealed pleomorphic populations of cell wall-deficient cells with ultrastructure morphology similar to that of a control stable L-form strain of S. aureus. The results demonstrate that expression of phenotypic methicillin resistance could be associated with cell wall deficiency in S. aureus strains and could underlie the phenomenon of heteroresistance.

Staphylococcus aureus produces autolysin-susceptible cell walls during growth in a high-NaCl and low-Ca2+ concentration medium

The growth of Staphylococcus aureus 209P becomes unusually sensitive to a high-NaCl concentration by decreasing the Ca2+ concentration in growth media, and cells either autolyze or transform into protoplast-like forms when grown standing in high-NaCl and low-Ca2+ concentration media below 37 C (Ochiai, T., Microbiol. Immunol. 43 (7): 705-709, 1999). To assess the role of Ca2+ in the salt tolerance of this organism, cells grown in the presence of different concentrations of Ca2+ were treated with boiling SDS, and their susceptibilities to crude autolysin (3 M LiCl extract of S. aureus 209P cells) were evaluated by turbidimetric assay and zymographic analysis. Susceptibilities of SDS-treated cells (SDS-cells) to crude autolysin were significantly influenced by Ca2+ concentration in the culture, and SDS-cells prepared from cultures grown in high-NaCl and high-Ca2+ concentration media exhibited marked resistance to crude autolysin when the assay system contained a high concentration of NaCl. On the contrary, SDS-cells prepared from cultures grown in high-NaCl and low-Ca2+ concentration media were rather susceptible to crude autolysin under the same assay conditions. A zymographic analysis revealed that the constitution of cell-associated autolysins was not influenced by the concentration of exogenous Ca2+. These results suggested that at least part of the mechanism of salt-induced autolysis in S. aureus 209P might be related to the synthesis of an autolysin susceptible cell wall.

Interaction of alveolar macrophages with Staphylococcus aureus and induction of microbial L-forms during infection in rats

In vivo cell interactions between Staphylococcus aureus and rat alveolar macrophages were investigated after intranasal inoculation during a 30-days period of examination. Some dynamic characteristics of microorganisms in the macrophages were examined by electron microscopy and acid phosphatase cytochemistry. It was found that at earlier infection intervals (days 3 and 7) the ingested cocci were sequestered in phagosomes and phagolysosomes and later many of the microbial cells were digested. An interesting finding was the intracellular appearance of cell wall-defective forms (L-forms) of S. aureus at later intervals (days 14 and 30 after challenge). Infection kinetics were evaluated by isolation and enumeration of colony-forming units of S. aureus from bronchoalveolar fluid and by assessment of blood and bronchoalveolar total and differential leukocyte counts. The results indicate that induction and survival of S. aureus L-forms may occur spontaneously in vivo. This phenomenon could explain some of the mechanisms, provoking the latent and relapsing lung infections.

Salt-Sensitive growth of Staphylococcus aureus: stimulation of salt-induced autolysis by multiple environmental factors

The growth of Staphylococcus aureus 209P became extremely sensitive to a high NaCl concentration following lowered temperature, reduced air-supply, and decreased Ca2+ concentration in the medium. Cells in high-NaCl and low-Ca2+ concentration media either autolyzed or transformed into protoplast-like forms during growth when grown standing below 37 degrees C. The abnormal growth, however, was invariably avoided by preliminary supplementation with polyanetholesulfonate (autolysin inhibitor) in the growth media. These results suggested that the autolytic activity of this organism was precisely controlled by multiple environmental factors such as ionic strength, temperature, air supply, and the concentration of Ca2+.