Factors influencing the resistance of biological monitors to ethylene oxide

The Influence of Sporulation Conditions on the Spore Coat Protein Composition of Bacillus subtilis Spores

Spores are of high interest to the food and health sectors because of their extreme resistance to harsh conditions, especially against heat. Earlier research has shown that spores prepared on solid agar plates have a higher heat resistance than those prepared under a liquid medium condition. It has also been shown that the more mature a spore is, the higher is its heat resistance most likely mediated, at least in part, by the progressive cross-linking of coat proteins. The current study for the first time assesses, at the proteomic level, the effect of two commonly used sporulation conditions on spore protein presence. ¹⁴N spores prepared on solid Schaeffer’s-glucose (SG) agar plates and ¹⁵N metabolically labeled spores prepared in shake flasks containing 3-(N-morpholino) propane sulfonic acid (MOPS) buffered defined liquid medium differ in their coat protein composition as revealed by LC-FT-MS/MS analyses. The former condition mimics the industrial settings while the latter conditions mimic the routine laboratory environment wherein spores are developed. As seen previously in many studies, the spores prepared on the solid agar plates show a higher thermal resistance than the spores prepared under liquid culture conditions. The ¹⁴N:¹⁵N isotopic ratio of the 1:1 mixture of the spore suspensions exposes that most of the identified inner coat and crust proteins are significantly more abundant while most of the outer coat proteins are significantly less abundant for the spores prepared on solid SG agar plates relative to the spores prepared in the liquid MOPS buffered defined medium. Sporulation condition-specific differences and variation in isotopic ratios between the tryptic peptides of expected cross-linked proteins suggest that the coat protein cross-linking may also be condition specific. Since the core dipicolinic acid content is found to be similar in both the spore populations, it appears that the difference in wet heat resistance is connected to the differences in the coat protein composition and assembly. Corroborating the proteomic analyses, electron microscopy analyses show a significantly thinner outer coat layer of the spores cultured on the solid agar medium.

Comparison of the properties of Bacillus subtilis spores made in liquid or on agar plates

AIMS

To compare the properties of the spores of Bacillus subtilis prepared in liquid and on plates.

METHODS AND RESULTS

The spores of B. subtilis were prepared at 37 degrees C using a nutrient exhaustion medium either in liquid or on agar plates. The levels of core water, dipicolinic acid (DPA) and small, acid-soluble spore proteins (SASP) were essentially identical in spores made in liquid or on plates. Spores prepared in liquid were killed approximately threefold more rapidly at 90 degrees C in water than the spores prepared on plates, and the spores prepared in liquid were more sensitive to nitrous acid and a diluted stable superoxidized water. Spores prepared in liquid also germinated more rapidly with several agents than those prepared on plates. Pellets of spores prepared on plates were darker than spores prepared in liquid, and spores prepared in liquid had more readily extracted coat protein. However, there were no major differences in the relative levels of individual coat proteins or the cross-linking of the coat protein GerQ in the two types of spores, although the inner membrane of spores prepared on plates had a higher ratio of anteiso- to iso-fatty acids.

CONCLUSIONS

The preparation in liquid yielded spores with some different properties than those made on agar plates. Spores made in liquid had lower resistance to heat and several chemicals, and germinated more readily with several agents. There were also differences in the composition of the inner membrane of spores prepared under these two conditions. However, there were no major differences in the levels of DPA, core water, SASP and individual coat proteins or the cross-linking of a coat protein in spores made in liquid and on plates.

SIGNIFICANCE AND IMPACT OF THE STUDY

This work demonstrates that the preparation method can affect the resistance and germination properties of bacterial spores, even if an identical medium and temperature are used. Evidence was also obtained consistent with the role of the inner membrane in spore resistance and germination, and that some factor in addition to core water, DPA and SASP content plays a role in spore resistance to wet heat.

Some factors affecting airborne survival of Pseudomonas fluorescens indoors

The effect of relative humidity (RH) on the airborne survival of a Pseudomonas fluorescens strain was studied at 20, 40, 60 and 80% RH indoors. The aero-stable spore of Bacillus subtilis subsp. niger, used as a tracer of physical losses was compared with a light scattering particle counter, as there were doubts about the reliability of the spore as tracer. The Rion counter was validated before use and found to give a good estimate of relative physical losses providing spray suspensions contained between 10(7) and 10(9) cfu ml-1 cells and that the humidity was not more than 80% RH. Pseudomonas fluorescens, strain P+S+, suspended in distilled water survived best at mid humidities and least at 80% RH. When suspended in 1% glycerol there was an apparent 'increase' in viability after an initial rapid reduction. This was thought to be due to delay in equilibration of glycerol in the cell membrane after concentration on dehydration. The cells were thought to be unstable and sensitive to the stress of rehydration before equilibration occurred. The findings are discussed in relation to Cox's theories of outer membrane damage on aerosolization.

Evaluation of ethylene oxide sterilization of tissue implants

The ability of ethylene oxide (ETO) gas to penetrate tissue matrices was evaluated using Bacillus subtilis var niger (NCTC 10073) spore strips sandwiched between sheets of various tissues exposed to ETO for varying periods. The decimal reduction times (min) obtained using commercially prepared spores were: 7.97 for naked spores, 7.34 between amnion, and 6.39 between amnio-chorion, indicating complete penetration of the matrix by the gas. Similar conclusions were reached using skin and dura mater. The possible protective effect of serum was evaluated by drying spores suspended in serum in different ways, followed by exposure to the gas. Decimal reduction times obtained were: 8.67 (dried at 37 degrees C); 8.09 (dried at 55 degrees C) and 8.97 (freeze-dried) confirming slight protection. However, in all cases sterilization was achieved following 100 min ETO exposure, giving a 2.5 times safety factor for the commercial ethylene oxide sterilizer.

The use of a model system to compare the efficiency of ultrasound and agitation in the recovery of Bacillus subtilis spores from polymer surfaces

The effectiveness of ultrasound and mechanical agitation for the recovery of Bacillus subtilis spores from pre-inoculated glass, polystyrene, polypropylene, polyethylene and polycarbonate surfaces was tested in the presence and absence of surfactant. Recovery from polymers was improved by the use of surfactant. Simple agitation, as frequently used in bioburden determination, is shown to be a poor method for the recovery of spores from hard surfaces. The use of ultrasound achieves recoveries over 90% only with glass, recoveries from polymers ranging from 40%-75%. Recoveries from polypropylene and polycarbonate showed poor reproducibilities. Agitation with Ballotini beads improved the reproducibility of recoveries from polymers and also in the majority of cases improved recovery levels.

The influence of water on the resistance of spores to inactivation by gaseous ethylene oxide

Standardized conditions for exposure to ethylene oxide were used to evaluate the resistance of spores dried for various times at different relative humidities and temperatures. Spores dried under conditions of high humidity exhibited low resistance to the sterilant, the resistance increasing as the relative humidity (RH) was decreased. Increasing the temperature of drying amplified this effect by reducing the time required for equilibration to a specific RH. Spores dried over a desiccant at 37 degrees C showed a slight rise followed by a fall in resistance. Spores maintained under these conditions for a long period of time increased in resistance. Spores rapidly dried by exposure to low RH, over a desiccant or at elevated temperature, dried unevenly resulting in a heterogeneous population with respect to ethylene oxide resistance. This was expressed as non-logarithmic survivor curves. The initial vacuum drawn influences resistance. The resistance of spores dried on aluminium foil increased as the pressure was reduced. The rate at which the pressure was reduced had little effect on resistance, except with highly desiccated spores. Dried spores held at different reduced pressures with humidification, showed no differences in resistance. The implications of these findings in relation to the operation of ethylene oxide sterilization cycles and the preparation and use of biological monitors is discussed.

A standardized monitor for the control of ethylene oxide sterilization cycles

The resistance of spores of B. subtilis var. niger produced in liquid synthetic medium and exposed to ethylene oxide on a nylon surface, has been shown to the almost identical to that for spores produced on a traditional solidified complex medium with exposure to the sterilant on aluminium foil. The use of short lengths of nylon tube as carriers allowed easy production and handling, with self-protection of the spore-bearing surface. Addition of a dye provided visual evidence of inoculation without affecting resistance to ethylene oxide. Such a monitor is suitable for use as a standardized biological challenge in routine ethylene oxide sterilization cycles.