The role of Serratia marcescens in soft contact lens associated ocular infections. A review

Serratia marcescens is a Gram negative rod which for a century and a half was considered a harmless saphrophyte. However, medical technology and the use of antibacterial agents have created ecological niches for this bacterium, which is now a medical problem. The bacterium is encountered in connection with contact lens keratitis, often associated with contaminated contact lens solutions. The concentrations of chlorhexidin and thiomersal required in contact lens solution to suppress the bacterium have been proved toxic to the eye. Modern contact lens solutions with biguanids have rapid killing kinetics, while in solutions with polyquaternium S. marcescens can survive in reduced numbers for up to 72 hours. The adherence of a specific isolate of Serratia to hydrogel lenses increased with decreased water content of the lenses. However, there has been no correlation between hydrophobicity markers or hemagglutinins and adherence to contact lenses or urinary tract epithelium. When handling medical plastic devices, such as contact lenses, strictly enforced hygiene remains the most important method to combat environmental bacteria such as Serratia marcescens.

DNA from Serratia marcescens confers a hydrophobic character in Escherichia coli

In order to determine whether hydrophobic surface properties of Serratia marcescens can be transferred to Escherichia coli, E. coli DH5 alpha cells were transformed by DNA fragments from S. marcescens RZ. Fifteen-hundred E. coli transformants were screened for adhesion to hexadecane and polystyrene. One transformant exhibited increased adhesion to hexadecane droplets, as well as altered kinetics of aggregation in the presence of ammonium sulfate. Western colony blotting revealed that antibodies raised against S. marcescens RZ recognized component(s) on the transformant outer surface.

Hemagglutination (fimbriae) and hydrophobicity in adherence of Serratia marcescens to urinary tract epithelium and contact lenses

The capacity of 59 isolates of Serratia marcescens, obtained from urinary tract infections, wounds, and contact lenses or their paraphernalia, to agglutinate erythrocytes from different animal species was tested. Three main patterns were found: mannose-sensitive agglutination of guinea-pig, fowl or horse erythrocyte; mannose-resistant agglutination of chicken or pigeon erythrocytes alone or in combination with mannose-sensitive agglutination; and no agglutination. Hemagglutination capacity was associated with isolates from urinary tract infection, but not with isolates associated with contact lenses. Adherence to human urinary tract epithelium did not correlate with the hemagglutination patterns nor with the origin of the isolates. Some strains of different hemagglutination pattern were selected for the study of hydrophobicity and adherence to contact lens polymers. Hydrophobicity, as determined by degree of partition in hexadecane and water (BATH-values), correlated neither with degree of adherence to contact lens polymers nor with the hemagglutination pattern. For a representative strain there was an excellent correlation (r2 = 0.98) between adherence and the water content (hydrophobicity) of the lens polymers. These results suggest that, as with tissues, other factors interact with hydrophobicity in causing adherence to plastics.

Surface layers of bacteria

Since bacteria are so small, microscopy has traditionally been used to study them as individual cells. To this end, electron microscopy has been a most powerful tool for studying bacterial surfaces; the viewing of macromolecular arrangements of some surfaces is now possible. This review compares older conventional electron-microscopic methods with new cryotechniques currently available and the results each has produced. Emphasis is not placed on the methodology but, rather, on the importance of the results in terms of our perception of the makeup and function of bacterial surfaces and their interaction with the surrounding environment.

Increased cell surface hydrophobicity of a Serratia marcescens NS 38 mutant lacking wetting activity

The cell surface hydrophobicity of Serratia marcescens appears to be an important factor in its adhesion to and colonization of various interfaces. The cell surface components responsible for mediating the hydrophobicity of S. marcescens have not been completely elucidated, but may include prodigiosin and other factors. In the present report we have investigated the potential role of serratamolide, an amphipathic aminolipid present on the surfaces of certain S. marcescens strains, in modulating cell surface hydrophobicity. The hydrophobic properties of a serratamolide-producing strain (NS 38) were compared with those of a serratamolide-deficient mutant (NS 38-9) by monitoring the kinetics of adhesion to hexadecane. Serratamolide production was monitored by thin-layer chromatography and the wetting activity of washed-cell suspensions on polystyrene. Wild-type NS 38 cells were far less hydrophobic than the serratamolide-deficient mutant cells were; the removal coefficients were 48 min-1 for the mutant, as compared with only 18 min-1 for the wild type. The data suggest that the presence of serratamolide on S. marcescens cells results in a reduction in hydrophobicity, presumably by blocking hydrophobic sites on the cell surface.

Droplet enrichment factors of pigmented and nonpigmented Serratia marcescens: possible selective function for prodigiosin

Drops produced by bursting bubbles provide a mechanism for the water-to-air transfer and concentration of matter. Bacteria can adsorb to air bubbles rising through bacterial suspensions and enrich the drops formed by the bubbles upon breaking, creating atmospheric biosols which function in dispersal. This bacterial enrichment can be quantified as an enrichment factor (EF), calculated as the ratio of the concentration of bacteria in the drop to that of the bulk bacterial suspension. Bubbles were produced in suspensions of pigmented (prodigiosin-producing) and nonpigmented cultures of Serratia marcescens. EFs for pigmented cultures were greater than EFs for nonpigmented cells. Pigmented cells appeared hydrophobic based on their partitioning in two-phase systems of polyethylene glycol 6000 and dextran T500. The surface hydrophobicity of pigmented cells may result from the hydrophobic nature of prodigiosin and could account for the greater ability of these bacteria to adsorb to air bubbles and enrich airborne droplets. Enhancement of the aerosolization of S. marcescens may be a selective function of the bacterial secondary metabolite prodigiosin.

Influence of Serratia marcescens Pigmentation on Cell Concentrations in Aerosols Produced by Bursting Bubbles

For eight strains of Serratia marcescens , increased cell concentrations were found in aerosols produced from bursting bubbles, with concentrations ranging from a maximum of ca. 80 times the bulk concentration for pigmented strains 4180, 933, and 274 to a minimum approximately equal to the bulk concentration for nonpigmented strain 8100. The increased cell concentration in the aerosol was suppressed when pigmented strains were grown at 37°C, a temperature at which the pigment prodigiosin is not synthesized, resulting in lower concentrations similar to those of nonpigmented strains. Strains that produce higher concentrations of prodigiosin after 1, 2, 4, and 8 days of growth show increasing concentrations in bubble-produced drops; duplicate cultures grown at 37°C did not show any increases. In four concurrent experiments, cells starved for 24 h showed greater concentrations than nonstarved cells for chromogenic strain NIMA, whereas for nonchromogenic strain WF, starved cells showed greater concentrations in three cases and a decreased concentration in the fourth. Bacterial concentrations in aerosol drops from bursting bubbles appear to be predominantly influenced by the surface condition of the bacterial cell.

Isolation of pigmented and nonpigmented mutants of Serratia marcescens with reduced cell surface hydrophobicity

Enrichment for nonhydrophobic mutants of Serratia marcescens yielded two types: (i) a nonpigmented mutant which exhibited partial hydrophobic characteristics compared with the wild type, as determined by adherence to hexadecane and polystyrene; and (ii) a pigmented, nonhydrophobic mutant whose colonies were translucent with respect to those of the wild type. The data suggest that the pronounced cell surface hydrophobicity of the wild type is mediated by a combination of several surface factors.

Starvation-Induced Effects on Bacterial Surface Characteristics

Changes in bacterial surface hydrophobicity, charge, and degree of irreversible binding to glass surfaces of seven marine isolates were followed during starvation. The degree of hydrophobicity was measured by hydrophobic interaction chromatography and by two-phase separation in a hexadecane-water system, whereas changes in charge were measured by electrostatic interaction chromatography. All isolates underwent the starvation-induced responses of fragmentation, which is defined as division without growth, and continuous size reduction, which results in populations with increased numbers of smaller cells. The latter process was also responsible for a significant proportion of the total drop in cell volume; this was observed by noting the biovolume (the average cell multiplied by the number of bacteria) of a population after various times of starvation. Four strains exhibited increases in both hydrophobicity and irreversible binding, initiated after different starvation times. The most hydrophilic and most hydrophobic isolates both showed a small increase in the degree of irreversible binding after only 5 h, followed by a small decrease after 22 h. Their hydrophobicity remained constant, however, throughout the entire starvation period. On the other hand, one strain, EF190, increased its hydrophobicity after 5 h of starvation, although the degree of irreversible binding remained constant. Charge effects could not be generally related to the increase in irreversible binding. Scanning electron micrographs showed a large increase in surface roughness throughout the starvation period for all strains that showed marked changes in physicochemical characteristics.

Bacterial activity at the air/water interface

By using substrate molecules of varying degrees of surface activity, we were able to measure some features of bacterial activity in the surface microlayers (SM) and in the subsurface (bulk) water. The fraction of active cells was determined by a combined microautoradiography-epifluorescence (ME) method. Measurements were made of(14)CO2 evolution to determine the rate of respiration. Results from in situ measurements showed no significant difference between fraction of active cells in the SM and in the bulk. This may be due to an exchange of bacteria between SM and bulk. This exchange was assessed by spreading a film of(3)H-palmitic acid on the surface and, after incubation, measuring the amount of labeled cells at the surface and in the bulk. Test bacteria showing a high accumulation at the surface also showed a low exchange between the 2 strata. When low concentrations of added(14)C-protein were used, the respiration measurements showed a lower value for bulk than for interface localized protein. At higher concentrations, the evolved(14)CO2 was the same whether the protein was mixed in the bulk or spread at the surface. When 2.4-12 ng·cm(-2) of(14)C-palmitic acid was spread on the surface, there was a linear relation between turnover time and amount of added substrate. At higher substrate concentrations there was a deviation from the straight line. Results are discussed in terms of the unique habitat found at an interface.

The hydrophobicity of bacteria - an important factor in their initial adhesion at the air-water interface

Bacteria isolated from the surface and the subsurface water at four stations along the Swedish west coast were assessed for their hydrophobicity with hydrophobic interaction chromatography (HIC). The surface bacteria were sampled by the Teflon sheet technique. [3H]-L-leucine metabolically labeled isolates were run on a column packed with Octyl-Sepharose CL-4B gel. The relative hydrophobicity of the bacteria was expressed as the ratio, g/e, between the radioactivity of the gel and the eluate. The results revealed a positive correlation between the degree of enrichment of bacteria at the surface and their hydrophobicity. The subsurface bacteria exhibited a broader spectrum of g/e-values than the surface bacteria. The initial adhesion of bacteria to the surface microlayer depends on several factors of which the hydrophobic interaction may be one of the most important.