Germinability and heat resistance of spores of Clostridium difficile strains

Investigating the transient and persistent effects of heat on Clostridium difficile spores

Purpose. Clostridium difficile spores are extremely resilient to high temperatures. Sublethal temperatures are associated with the 'reactivation' of dormant spores, and are utilized to maximize C. difficile spore recovery. Spore eradication is of vital importance to the food industry. The current study seeks to elucidate the transient and persisting effects of heating C. difficile spores at various temperatures.Methods. Spores of five C. difficile strains of different ribotypes (001, 015, 020, 027 and 078) were heated at 50, 60 and 70-80 °C for 60 min in phosphate-buffered saline (PBS) and enumerated at 0, 15, 30, 45 and 60 min. GInaFiT was used to model the kinetics of spore inactivation. In subsequent experiments, spores were transferred to enriched brain heart infusion (BHI) broths after 10 min of 80 °C heat treatment in PBS; samples were enumerated at 90 min and 24 h.Results. The spores of all strains demonstrated log-linear inactivation with tailing when heated for 60 min at 80 °C [(x̄=7.54±0.04 log₁₀ vs 4.72±0.09 log₁₀ colony-forming units (c.f.u.) ml^{- 1}; P<0.001]. At 70 °C, all strains except 078 exhibited substantial decline in recovery over 60 min. Interestingly, 50 °C heat treatment had an inhibitory effect on 078 spore recovery at 0 vs 60 min (7.61±0.06 log₁₀ c.f.u. ml^{- 1} vs 6.13±0.05 log₁₀ c.f.u. ml^{- 1}; P<0.001). Heating at 70/80 °C inhibited the initial germination and outgrowth of both newly produced and aged spores in enriched broths. This inhibition appeared to be transient; after 24 h vegetative counts were higher in heat-treated vs non-heat-treated spores (x̄=7.65±0.04 log₁₀ c.f.u. ml^{- 1} vs 6.79±0.06 log₁₀ c.f.u. ml^{- 1}; P<0.001).Conclusions. The 078 spores were more resistant to the inhibitory effects of higher temperatures. Heat initially inhibits spore germination, but the subsequent outgrowth of vegetative populations accelerates after the initial inhibitory period.

Thermal Resistance of Clostridium difficile Spores in Peptone Water and Pork Meat

The thermal resistance of four strains of Clostridium difficile spores (three hypervirulent and one nonhypervirulent) in peptone water (PW) and pork meat was evaluated individually at 70, 75, 80, 85, and 90°C using two recovery methods (taurocholate and lysozyme). PW or meat was inoculated with C. difficile spores and mixed to obtain ca. 5.0 log CFU/ml or 4.0 log CFU/g, respectively. The D-values of C. difficile spores in PW ranged from 7.07 to 22.14 h, 1.42 to 3.82 h, 0.35 to 0.59 h, 4.93 to 5.95 min, and 1.16 to 1.76 min at 70, 75, 80, 85, and 90°C, respectively, for the four strains using the taurocholate method. The D-values of the respective C. difficile spores were greater (P ≤ 0.05) using the lysozyme method, especially at higher temperatures (85 and 90°C). Greater thermal resistance of C. difficile spores was observed in meat than in PW using the lysozyme method. Hypervirulence of the C. difficile strains was not associated with greater thermal resistance in meat. The z-values for C. difficile spores in meat were between 6.21 and 7.21°C, and they were 11.24 and 12.12°C using the taurocholate and the lysozyme recovery methods, respectively. The D- and z-values of C. difficile spores were greater in both PW and pork than the values reported in the literature. C. difficile spores can survive traditional cooking or thermal processing practices and potentially grow in cooked, ready-to-eat products. The use of effective methods to recover heat-injured spores is necessary to obtain accurate thermal destruction parameters for C. difficile spores.

Inactivation of Clostridium difficile spores by microwave irradiation

Spores are a potent agent for Clostridium difficile transmission. Therefore, factors inhibiting spores have been of continued interest. In the present study, we investigated the influence of microwave irradiation in addition to conductive heating for C. difficile spore inactivation in aqueous suspension. The spores of 15 C. difficile isolates from different host origins were exposed to conductive heating and microwave irradiation. The complete inhibition of spore viability at 10(7) CFU/ml was encountered following microwave treatment at 800 W for 60 s, but was not observed in the conductive-heated spores at the same time-temperature exposure. The distinct patterns of ultrastructural alterations following microwave and conductive heat treatment were observed and the degree of damages by microwave was in the exposure time-dependent manner. Microwave would therefore be a simple and time-efficient tool to inactivate C. difficile spores, thus reducing the risk of C. difficile transmission.

A possible route for foodborne transmission of Clostridium difficile?

Spores of toxigenic Clostridium difficile and spores of food-poisoning strains of Clostridium perfringens show a similar prevalence in meats. Spores of both species are heat resistant and can survive cooking of foods. C. perfringens is a major cause of foodborne illness; studies are needed to determine whether C. difficile transmission by a similar route is a cause of infection.

Performance of chromID Clostridium difficile agar compared with BBL C. difficile selective agar for detection of C. difficile in stool specimens

We evaluated the performance of a new chromogenic medium for detection of Clostridium difficile, chromID C. difficile agar (CDIF; bioMérieux, France), by comparison with BBL C. difficile Selective Agar (CDSA; Becton Dickinson and Company, USA). After heat pre-treatment (80℃, 5 min), 185 diarrheal stool samples were inoculated onto the two media types and incubated anaerobically for 24 hr and 48 hr for CDIF and for 48 hr and 72 hr for CDSA. All typical colonies on each medium were examined by Gram staining, and the gram-positive rods confirmed to contain the tpi gene by PCR were identified as C. difficile. C. difficile was recovered from 36 samples by using a combination of the two media. The sensitivity with CDIF 48 hr was highest (100%) and was significantly higher than that with CDIF 24 hr (58.3%; P<0.001), because samples with a low burden of C. difficile tended to require prolonged incubation up to 48 hr (P<0.001). The specificity of CDIF 24 hr and CDIF 48 hr (99.3% and 90.6%, respectively) was significantly higher than that of CDSA 48 hr and CDSA 72 hr (72.5% and 67.1%, respectively; P<0.001). CDIF was effective for detecting C. difficile in heat-pretreated stool specimens, thus reducing unnecessary testing for toxin production in non-C. difficile isolates and turnaround time.

Sensitive and selective culture medium for detection of environmental Clostridium difficile isolates without requirement for anaerobic culture conditions

Effective and easy-to-use methods for detecting Clostridium difficile spore contamination would be useful for identifying environmental reservoirs and monitoring the effectiveness of room disinfection. Culture-based detection methods are sensitive for detecting C. difficile, but their utility is limited due to the requirement of anaerobic culture conditions and microbiological expertise. We developed a low-cost selective broth medium containing thioglycolic acid and l-cystine, termed C. difficile brucella broth with thioglycolic acid and l-cystine (CDBB-TC), for the detection of C. difficile from environmental specimens under aerobic culture conditions. The sensitivity and specificity of CDBB-TC (under aerobic culture conditions) were compared to those of CDBB (under anaerobic culture conditions) for the recovery of C. difficile from swabs collected from hospital room surfaces. CDBB-TC was significantly more sensitive than CDBB for recovering environmental C. difficile (36/41 [88%] versus 21/41 [51%], respectively; P = 0.006). C. difficile latex agglutination, an enzyme immunoassay for toxins A and B or glutamate dehydrogenase, and a PCR for toxin B genes were all effective as confirmatory tests. For 477 total environmental cultures, the specificity of CDBB-TC versus that of CDBB based upon false-positive yellow-color development of the medium without recovery of C. difficile was 100% (0 false-positive results) versus 96% (18 false-positive results), respectively. False-positive cultures for CDBB were attributable to the growth of anaerobic non-C. difficile organisms that did not grow in CDBB-TC. Our results suggest that CDBB-TC provides a sensitive and selective medium for the recovery of C. difficile organisms from environmental samples, without the need for anaerobic culture conditions.

Molecular and microbiological characterization of Clostridium difficile isolates from single, relapse, and reinfection cases

In this study, we investigated the correlation between the microbiological characteristics of Clostridium difficile clinical isolates and the recurrence of C. difficile-associated disease (CDAD). Twenty C. difficile isolates recovered from 20 single infection cases and 53 isolates from 20 recurrent cases were analyzed by pulsed-field gel electrophoresis (PFGE) and PCR ribotyping, and the cytotoxicity, antimicrobial susceptibility, and sporulation/germination rates of the isolates were examined. Recurrent cases were divided into relapse or reinfection cases by the results of C. difficile DNA typing. Among the 20 recurrent cases, 16 cases (80%) were identified to be relapse cases caused by the initial strain and the remaining 4 cases (20%) were identified to be reinfection cases caused by different strains. All 73 isolates were susceptible to both vancomycin and metronidazole, but resistance against clindamycin, ceftriaxone, erythromycin, and ciprofloxacin was found in 87.7%, 93.2%, 87.7%, and 100% of the isolates, respectively. No correlations between DNA typing group, cytotoxicity, and sporulation rate of isolates and infection status, i.e., single, relapse, or reinfection, were observed. However, the isolates recovered from relapse cases showed a significantly higher germination rate when incubated in medium lacking the germination stimulant sodium taurocholate. These results indicate that the germination ability of C. difficile may be a potential risk factor for the recurrence of CDAD.

Infection control measures to limit the spread of Clostridium difficile

Clostridium difficile-associated diarrhoea (CDAD) presents mainly as a nosocomial infection, usually after antimicrobial therapy. Many outbreaks have been attributed to C. difficile, some due to a new hyper-virulent strain that may cause more severe disease and a worse patient outcome. As a result of CDAD, large numbers of C. difficile spores may be excreted by affected patients. Spores then survive for months in the environment; they cannot be destroyed by standard alcohol-based hand disinfection, and persist despite usual environmental cleaning agents. All these factors increase the risk of C. difficile transmission. Once CDAD is diagnosed in a patient, immediate implementation of appropriate infection control measures is mandatory in order to prevent further spread within the hospital. The quality and quantity of antibiotic prescribing should be reviewed to minimise the selective pressure for CDAD. This article provides a review of the literature that can be used for evidence-based guidelines to limit the spread of C. difficile. These include early diagnosis of CDAD, surveillance of CDAD cases, education of staff, appropriate use of isolation precautions, hand hygiene, protective clothing, environmental cleaning and cleaning of medical equipment, good antibiotic stewardship, and specific measures during outbreaks. Existing local protocols and practices for the control of C. difficile should be carefully reviewed and modified if necessary.

Clostridium difficile colonization in healthy adults: transient colonization and correlation with enterococcal colonization

The aim of the present study was to investigate the colonization status of Clostridium difficile in healthy individuals. In total, 139 healthy adults from two study groups were examined at intervals of 3 months. Among the 18 positive subjects, the number of subjects from whom C. difficile was isolated once, twice, three times or four times was 10 (55.6%), three (16.7%), two (11.1%) and three (16.7%), respectively. In the student group, different subjects were colonized by different PCR ribotype/PFGE types. However, the same PCR ribotype/PFGE types of C. difficile were isolated from different subjects in the employee group, indicating that cross-transmission may have occurred in this group. Continuous colonization by the same PCR ribotype/PFGE type was only observed in three subjects. C. difficile-positive subjects were significantly more densely colonized by enterococci (P<0.05) than C. difficile-negative subjects: subjects that were found to be C. difficile-positive three or four times appeared to have higher concentrations of enterococci. The present results demonstrate that, although colonization by a C. difficile strain is transient in many cases, there are healthy individuals that are colonized persistently by C. difficile. They also suggest that dense colonization of the intestine by enterococci may be associated with C. difficile colonization.

The abattoir source of culturable psychrophilic Clostridium spp. causing 'blown pack' spoilage of vacuum-packed chilled venison

AIMS

To identify the abattoir source(s) of culturable psychrophilic clostridia causing 'blown pack' spoilage of vacuum-packed chilled meats.

METHODS AND RESULTS

Psychrophilic and psychrotolerant clostridia were isolated from hides, faeces and tonsils of deer slaughter stock, and from a meat plant environment. The isolates were differentiated using restriction fragment length polymorphism analysis of the 16S rDNA gene (PCR-RFLP) and 16S-23S rDNA internal transcribed spacer (ITS) analysis. PCR-RFLP group I clostridia were found to have restriction patterns indistinguishable from the patterns of 'blown pack'-causing Clostridium gasigenes DB1A(T) and R26. Gas production in packs inoculated with vegetative cells of PCR-RFLP group I clostridia was first evident after 14 days at 2 degrees C. The prevalence of these clostridia was similar in hide and faecal samples from slaughter animals, but these micro-organisms were absent from tonsils and the meat plant environment. Banding patterns of PCR-RFLP group II clostridia showed some cross-similarity with patterns of the 'blown pack'-causing micro-organism Cl. estertheticum DSM 8809(T) and Cl. estertheticum-like meat strains. The majority of clostridia in PCR-RFLP group II were found in the faeces of slaughter animals. Isolates representing PCR-RFLP group II did not, however, produce gas in vacuum packs stored at 2 degrees C for 84 days.

CONCLUSIONS

The data suggest that soil particles attached to hide or present in faeces are the most probable primary reservoir from which 'blown pack' clostridia are introduced onto carcasses. Therefore, dressing procedure hygiene remains paramount in order to control the spread of psychrophilic Clostridium spp. in a meat plant.

SIGNIFICANCE AND IMPACT OF THE STUDY

The paper provides information critical for controlling 'blown pack' spoilage in meat processing plants. It reports on the use of molecular techniques for determination of abattoir sources of 'blown pack'-causing clostridia.

Value of lysozyme agar incorporation and alkaline thioglycollate exposure for the environmental recovery of Clostridium difficile

Clostridium difficile is an increasingly prevalent nosocomial pathogen. Environmental contamination by spores is believed to be a major factor propagating the spread of C. difficile. Various approaches including the use of bile salts have been described to enhance the recovery of C. difficile from clinical and environmental specimens. We found that lysozyme (5 mg/L) incorporated into a selective medium containing bile salts significantly increased the recovery of C. difficile from swabs of 197 environmental sites (11% versus 24% samples positive, P< 0.01). Furthermore, in a separate series of experiments additional use of cooked meat broth enrichment significantly enhanced the recovery of C. difficile (35% versus 45%, P = 0.009). Conversely, we found that pre-exposure to alkaline thioglycollate did not improve the yield of C. difficile. Lysozyme incorporation markedly increases the recovery of C. difficile from environmental samples probably by stimulation of spore germination. Our findings suggest that previous attempts to determine the level of environmental C. difficile contamination have markedly underestimated the true prevalence of this pathogen.

Effect of isoleucine on toxin production by Clostridium difficile in a defined medium

Supplementation of a carbohydrate-free minimal medium with a high level (100 mM) of histidine, methionine, valine, isoleucine, proline and leucine, in particular isoleucine, markedly increased toxin production by Clostridium difficile VPI 10463. The effect of isoleucine was further examined. Increasing the concentration of isoleucine from 20 to 100 mM remarkably increased toxin production, while bacterial growth decreased gradually. Amino acid analysis of the culture revealed that, at 100 mM isoleucine, consumption of isoleucine was remarkably increased. During the incubation period when toxin titers increased markedly but bacterial growth was declining, isoleucine, leucine and cysteine were taken up preferentially and alanine and cystathionine, which were not found at 1 mM isoleucine, were produced in large quantities. These findings suggest that isoleucine may play an important role in toxin production by C. difficile and that alanine and cystathionine production may be co-regulated with the toxin production in the absence of fermentable carbohydrates.

Influence of culture media on the recovery of psychrotrophic Clostridium spp. associated with the spoilage of vacuum-packed chilled meats

This study was undertaken to determine the influence of culture media on the quantitative recovery of vegetative cells and spores of psychrotrophic Clostridium spp. associated with the spoilage of chilled meats. For recovery of vegetative cells and spores (presumptive counts), 24 h or 48 h broth cultures in Peptone Yeast Extract Glucose Starch (PYGS) broth were used; for spore counts, concentrated spore suspensions derived from 35-day cultures were used. For presumptive counts, seven non-selective and eight selective media were tested. Recovery of psychrotrophic clostridia with optimum growth temperatures between 15 degrees C and 20 degrees C was best with non-selective media, such as Peptone Yeast Extract Glucose Starch (PYGS) agar with lysozyme; recovery of clostridia with growth optima between 25 degrees C and 30 degrees C was best with selective media, such as Shahidi Ferguson Perfringens (SFP) agar. For organisms with the lower optimum growth temperature (heat-sensitive group) spore recovery after heat treatment (80 degrees C for 10 min) was best if a 2 M (pH 10) thioglycollate treatment (10 min at 45 degrees C) was used, followed by plating onto a lysozyme- or egg-yolk-containing medium. For organisms with the higher optimum growth temperature (heat-resistant group), spore recovery was best on Glucose Starch agar without added lysozyme. The diversity among psychrotrophic Clostridium spp. associated with chilled meat spoilage precludes the identification of a single 'best' recovery medium or technique. Consequently, a variety of complementary selective techniques and media must be used if comprehensive recovery is to be assured.

Factors affecting growth from heat-treated spores of non-proteolytic Clostridium botulinum

Heat treatment of spores of non-proteolytic Clostridium botulinum at 85°C for 120 min followed by enumeration of survivors on a medium containing lysozyme resulted in a 4.1 and 4.8 decimal reduction in numbers of spores of strains 17B (type B) and Beluga (type E), respectively. Only a small proportion of heated spores formed colonies on medium containing lysozyme; this proportion could be increased by treatments designed to increase the permeability of heated spores. The results indicate that the germination system in spores of non-proteolytic Cl. botulinum was destroyed by heating, that lysozyme could replace this germination system, and that treatments that increased the permeability of the spore coat could increase the proportion of heated spores that germinated on medium containing lysozyme. These results are important in relation to the assessment of heat-treatments required to reduce the risk of survival and growth of non-proteolytic Clostridium botulinum in processed (pasteurized) refrigerated foods for extended storage.

Effect of various sodium taurocholate preparations on the recovery of Clostridium difficile spores

The effect of four sodium taurocholate preparations, which are easily available in Japan, on recovery of Clostridium difficile spores was examined. All preparations, except for one, enabled the recovery of nearly all spores counted microscopically. Moreover, by using 69 toxigenic and 34 nontoxigenic C. difficile strains, the relationship between the recovery of spores in the medium with sodium taurocholate and toxigenicity of C. difficile was analyzed. It was noted that the number of strains with recovery rate of more than 70% was greater in toxigenic strains than in nontoxigenic strains, suggesting a more abundant recovery of toxigenic C. difficile strains in the presence of sodium taurocholate.