Role of adenine in pathogenesis of Mycoplasma pneumoniae infections of tracheal epithelium

Nuclease activity: an exploitable biomarker in bacterial infections

INTRODUCTION

In the increasingly challenging field of clinical microbiology, diagnosis is a cornerstone whose accuracy and timing are crucial for the successful management, therapy, and outcome of infectious diseases. Currently employed biomarkers of infectious diseases define the scope and limitations of diagnostic techniques. As such, expanding the biomarker catalog is crucial to address unmet needs and bring about novel diagnostic functionalities and applications.

AREAS COVERED

This review describes the extracellular nucleases of 15 relevant bacterial pathogens and discusses the potential use of nuclease activity as a diagnostic biomarker. Articles were searched for in PubMed using terms: "nuclease", "bacteria", "nuclease activity" or "biomarker". For overview sections, original and review articles between 2000 and 2019 were searched for using terms: "infections", "diagnosis", "bacterial", "burden", "challenges". Informative articles were selected.

EXPERT OPINION

Using the catalytic activity of nucleases offers new possibilities compared to established biomarkers. Nucleic acid activatable reporters in combination with different transduction platforms and delivery methods can be used to detect disease-associated nuclease activity patterns in vitro and in vivo for prognostic and diagnostic applications. Even when these patterns are not obvious or of unknown etiology, screening platforms could be used to identify new disease reporters.

Interaction of Mycoplasma pneumoniae with HeLa cells

The susceptibility of HeLa cells to Mycoplasma pneumoniae-induced injury was examined. Infections were initiated with relatively low mycoplasma doses, carried out in a culture medium incapable of supporting M. pneumoniae replication in the absence of host cells, and monitored for up to 10 days. Under these conditions, a time- and dose-dependent decline in the number of viable host cells compared with that of uninfected controls was observed. The effect of M. pneumoniae infection on host cell macromolecular synthesis was also evaluated. At high doses of infection, synthesis of both protein and RNA declined rapidly relative to that in control cells. At lower doses there was a biphasic response in protein synthesis, which was substantially lower than that in the uninfected control by day 1 postinfection, returned to control levels by day 4 postinfection, and was again less than that in control cells by day 7 postinfection. In contrast, no transient recovery was observed in RNA synthesis, which declined very gradually over 7 days in infected HeLa cells compared with that in uninfected control cells. The ability of HeLa cells to support the proliferation of M. pneumoniae under these experimental conditions was demonstrated by quantitation of mycoplasma CFU in the nonpermissive medium in the presence or absence of HeLa cells. A negligible increase in the number of M. pneumoniae was observed over 4 days when HeLa cells were absent, while CFU increased by almost 20-fold when M. pneumoniae was cultured in the presence of HeLa cells. The susceptibility and response in macromolecular synthesis in M. pneumoniae-infected HeLa cells differed from that recently described for a nontransformed culture of hamster trachea epithelial cells under the same experimental conditions (Y.-Y. Chen and D.C. Krause, Infect. Immun. 56: 570-576, 1988), underscoring the importance of the choice of host cell for in vitro modeling of M. pneumoniae pathogenesis.

Parasitism of hamster trachea epithelial cells by Mycoplasma pneumoniae

Hamster trachea epithelial (HTE) cells were employed as a model system for Mycoplasma pneumoniae pathogenesis. To more closely mimic natural infection, M. pneumoniae was forced to rely upon host cells (as opposed to the growth medium) for nutrients, and infections were initiated with relatively low mycoplasma doses and monitored for extended time periods. A time- and dose-dependent decline in the viability of infected cells was observed; however, viability never declined below 50% of that in uninfected controls. Protein and RNA synthesis actually increased above control levels in infected cells, despite a concomitant decrease in viability. This response was pronounced at higher multiplicities of infection but was only transient at lower doses. In parallel studies in which a culture medium capable of supporting M. pneumoniae growth was used, loss of viability was accelerated. With a low-dose infection a transient increase followed by a precipitous decline in macromolecular synthesis was observed, relative to that in uninfected controls. At higher doses, however, macromolecular synthesis decreased dramatically and in proportion to the loss of viability. The requirement for HTE cells for mycoplasma growth under the experimental culture conditions was demonstrated by quantitating viable mycoplasmas in the culture medium in the presence or absence of HTE cells over 4 days. The increase in mycoplasma number was negligible in the absence of HTE cells, while a 30-fold increase was observed in the presence of HTE cells. These findings demonstrate the feasibility of long-term, low-dose studies of M. pneumoniae pathogenesis with trachea epithelial cells and a nonpermissive culture medium. This experimental system should facilitate the elucidation of the mechanism(s) responsible for host cell injury, and perhaps reveal how host cells respond to infection.

Enzymatic activities in cell fractions of mycoplasmalike organisms purified from aster yellows-infected plants

Mycoplasmalike organisms (MLOs), purified from aster yellows-infected plants were osmotically lysed, and the membranes were separated from the cytoplasmic fraction through differential centrifugation. Electron microscopic examinations of sections of the purified MLOs and the isolated membranes showed pleomorphic bodies and unit membranous empty vesicles, respectively. Cell fractions were tested for NADH oxidase, NADPH oxidase, ATPase, RNase, DNase, and p-nitrophenyl phosphatase activity. NADH oxidase and ATPase were confined to the membrane fraction and NADPH oxidase to the cytoplasmic fraction of the MLOs. para-Nitrophenyl phosphatase, RNase, and DNase activities were detected in both membrane and cytoplasmic fractions, but p-nitrophenyl phosphatase and RNase appeared to be associated with membranes and DNase with the cytoplasmic fraction. Glucose-6-phosphate dehydrogenase was found in the cytoplasmic fraction of the MLO cells. Our findings on the distribution of enzymes in MLO cells and cell fractions are the first basic documentation on nonhelical, nonculturable microbes parasitic to plants.

Differential cytopathogenicity accompanying Mycoplasma pneumoniae infection of human lung fibroblasts maintained in newborn bovine serum or fetal bovine serum

MRC-5 human lung fibroblasts maintained in Eagle's basal medium (BME) with either 10% fetal bovine serum (FBS) or 10% newborn bovine serum (NBS) did not respond identically to infection by Mycoplasma pneumoniae. Fibroblasts grown in NBS did not develop any cytopathic effect (CPE) when infected with M. pneumoniae, whereas those maintained in FBS developed a pronounced CPE. There was also a difference in sensitivity to infection for fibroblasts maintained in the two sera before the infection. Fibroblasts maintained in NBS, then transferred to FBS 48 h before infection, were still less sensitive to M. pneumoniae infection than cells maintained constantly in FBS. Mycoplasma pneumoniae attached comparably to the fibroblasts grown in the two sera, so the differences in CPE development could not be attributed to differences in mycoplasma attachment. Measurements of DNA, RNA, and protein syntheses of the fibroblasts grown in NBS and FBS indicate that the cells in NBS were growing more rapidly than those in FBS. A determination of the doubling times shows that the doubling time of cells in NBS was 44 h, whereas that of cells in FBS was 51 h. Polyacrylamide gel electrophoresis of samples of NBS and FBS showed significant differences in serum protein composition. The NBS had several protein bands that were lacking in the FBS. This study demonstrates the importance of serum effects in the study of M. pneumoniae infection.

De novo purine synthesis, purine salvage, and DNA synthesis in normal and Lesch-Nyhan fibroblasts infected with Mycoplasma pneumoniae

The effects of Mycoplasma pneumoniae on host cell metabolism were studied by using two types of host cells, MRC-5 human lung fibroblasts, a normal cell line, and Lesch-Nyhan fibroblasts, a cell line deficient in hypoxanthine-guanine phosphoribosyl transferase (EC 2.4.2.8). The susceptibilities of the two cell types were determined by infecting the cells with M. pneumoniae at different multiplicities of infection (MOI). Our data indicate that the Lesch-Nyhan cells were four times more susceptible to damage by M. pneumoniae than the MRC-5 cells. The effects of different MOIs (10 and 50) on de novo purine synthesis. DNA synthesis, and the development of a cytopathic effect were determined. In both cell types, the higher MOI inhibited de novo purine synthesis to a greater extent than the lower MOI. This correlated closely with the cytopathic effect which developed in the monolayers (i.e., the more the inhibition of de novo purine synthesis, the greater the cytopathic effect which developed). In the Lesch-Nyhan cells, DNA synthesis was completely inhibited by the high MOI, whereas in the MRC-5 cells, DNA synthesis was stimulated by the high MOI. In the MRC-5 cells infected with M. pneumoniae, purine salvage activity increased, as indicated by an increase in adenosine deaminase (EC 3.5.4.4) activity. These data indicate that M. pneumoniae alters host cell metabolism, particularly the nucleic acid metabolic pathways. This may explain in part the mechanism of pathogenesis of M. pneumoniae infection.

Alterations in nucleotide content of human lung fibroblasts infected with Mycoplasma pneumoniae

The nucleotide content of normal MRC-5 human lung fibroblasts and fibroblasts infected with Mycoplasma pneumoniae PI 1428 was determined. Nucleotides from control and infected fibroblasts were extracted with 5% trichloracetic acid. After neutralization of the extracts, the nucleotides in the extracts were separated by anion-exchange chromatography. Significant differences were found between the nucleotide content of the control and infected cells. Nucleotide triphosphate levels were twofold higher in the control fibroblasts than in the infected fibroblasts 4 h after the initiation of infection. At the same time, nucleotide diphosphate and monophosphate levels were higher in the infected fibroblasts than in the control fibroblasts. Determination of the energy charge ratio for each set of nucleotides (adenosine, guanosine, cytidine, and uridine) demonstrated a shift of nucleotide content in the infected fibroblasts. Immediately after infection, the energy charge for each set of nucleotides was higher for the control fibroblasts than it was for the infected fibroblasts. This pattern continued throughout the infection period with only minor exceptions. The work presented here indicates a loss of energy charge in fibroblasts infected with M. pneumoniae and may help to explain some of the metabolic changes and cell damage which accompany infection.

Properties of the nucleases of mollicutes

Extracts of the Mollicutes Acholeplasma equifetale, Acholeplasma laidlawii B, Mycoplasma arthritidis. Mycoplasma pulmonis, and Mycoplasma pneumoniae had DNase and endonuclease activity. A. laidlawii B had at least two peaks of DNase activity in sucrose gradients with sedimentation coefficients of 3.1S and 4.3S. These fractions also had endonuclease activity with different substrate specificities. A. laidlawii B may have more than two peaks of endonuclease activity in sucrose gradients.

Scanning electron microscopy of Mycoplasma pneumoniae on the membrane of individual ciliated tracheal cells

Cell monolayer cultures were prepared from hamster tracheal explants by a collagenase exposure and subsequent incubation in Waymouth's MAB 87/3 medium. The epithelial outgrowth occurred on glass cover slips. Cilia on the monolayers continued to beat normally after the "parent" explant was removed. Monolayer cultures infected with Mycoplasma pneumoniae had significant amounts of attachment. A morphological analysis of the attachment was conducted with scanning electron microscopy. Clusters, cocci, and filaments of M. pneumoniae all attached to the epithelial cells, but the filaments were especially common. Mycoplasmas were seen in association with both ciliated and nonciliated cell membranes. On ciliated cells, mycoplasmas were on the ciliary strands and on the cell membrane. When located immediately adjacent to or in between cilia, mycoplasmas were oriented vertically with the constricted attachment tip oriented down toward the host cell membrane. When located more than a micron away from the ciliary fibers, mycoplasmas lay horizontally along the epithelial cell membrane. The photographic data suggest that clusters or "sperules" of mycoplasmas may liberate individual mycoplasmas that attach to the cell membrane. It appears that the receptor sites for M. pneumoniae are rather uniformly distributed along the ciliated cell membrane, and are not restricted to the interciliary areas.

Antimycoplasmal activity of dimethylphenols in a tracheal explant culture system

Mycoplasma pneumoniae induces pneumonia-like symptoms in hamsters and causes ciliostasis and cytonecrosis in hamster tracheal explants. 2,4-Dimethylphenol and, to a lesser extent, its 2,3-, 2,5-, and 2,6-dimethylphenol isomers protected tracheal explants from these changes after exposure to virulent M. pneumoniae strain PI 1428. The effect was concentration, time, and isomer dependent. At concentrations of 10(-9) M or greater, 2,4-dimethylphenol completely prevented the morphological (loss of ciliated cells) and biochemical (decreased dehydrogenase activity) changes normally observed after exposure to M. pneumoniae. Apparently, 2,4-dimethylphenol interfered with an early event in the infection process. Complete protection required that it be present during the first 2 h of exposure of the explants to the infecting mycoplasmas. These xylenols may prove to be useful tools for helping to define the mechanisms of pathogenesis in certain respiratory infections.

Role of cell-associated pathogen metabolism in infection of tracheal explants by Mycoplasma pneumoniae

The effect of environmental conditions on the relative pathogenicity of Mycoplasma pneumoniae for hamster tracheal explants was investigated. Organisms from the early stages of the growth cycle (e.g., day 1 to 2) were more effective in the induction of ciliostasis than were older cultures. Both the degree of ciliostasis and the speed of onset were affected. The type of explant culture medium also affected pathogenic potential. M. pneumoniae infection produced significantly greater ciliostasis and cytonecrosis in a "permissive" medium, i.e., one capable of supporting mycoplasma metabolism and replication, than in a "nonpermissive" medium. However, no adenine protection effect could be detected under permissive conditions, though it was quite striking when a nonpermissive medium was used as the post-infection explant medium. This suggests that the cell damage noted under permissive conditions may result from processes distinct from those operative in the actual host-parasite cellular interaction.

Ciliated respiratory epithelial monolayers: new model for Mycoplasma pneumoniae infection

Hamster respiratory epithelial cells were cultured in a monolayer format, and 20% of the cells were ciliated. Mycoplasma pneumoniae attached to the epithelial cells in a neuraminidase-specific fashion and induced ciliostasis and cytonecrosis.