Study Shows Bacillus cereus Persists on Bakery Food-Contact Surfaces After Sanitation, Describes Biofilms’ Influence
A new study has found that Bacillus cereus can persist on bakery food-contact surfaces following routine cleaning and sanitation, with biofilm formation, organic residues, and surface material influencing tolerance to disinfectants. Among four disinfectants evaluated, chlorine dioxide (ClO₂) and peracetic acid (PAA) demonstrated the greatest antimicrobial efficacy, although ClO₂ generally performed better under organic residue-contaminated conditions.
Published in the Journal of Agriculture and Food Research, the study was conducted by researchers at Prince of Songkla University in Thailand. The researchers investigated microbial contamination on food-contact equipment at a bakery processing facility following routine sanitation, assessed the biofilm-forming ability of recovered B. cereus isolates, and compared disinfectant efficacy against planktonic and biofilm-associated cells on stainless steel and polypropylene surfaces.
B. cereus Detected on Bakery Equipment After Sanitation
The researchers collected 44 surface samples from 11 pieces of food-contact equipment across four sampling events. B. cereus was the only foodborne pathogen detected among those investigated, which also included Clostridium perfringens, Escherichia coli, Staphylococcus aureus, and Salmonella spp.
B. cereus was recovered from stainless steel and plastic equipment, with maximum concentrations of 3.11 and 3.84 log colony forming units (CFU) per 50 square centimeters (cm²), respectively. Total viable counts on several surfaces exceeded the microbiological limit established by Thailand's Department of Medical Sciences.
Of 51 B. cereus isolates recovered, four (7.8 percent) exhibited weak biofilm-forming ability. Further testing revealed that low biofilm biomass did not necessarily correspond to low numbers of viable cells. Despite low biomass measurements, viable-cell counts ranged from 4.53 to 6.05 log CFU/4.5 cm² on stainless steel and polypropylene surfaces.
The researchers suggested that sanitation assessments relying solely on biofilm biomass measurements may underestimate microbial persistence on food-contact surfaces.
Chlorine Dioxide Demonstrated Strongest Antibiofilm Activity
The researchers evaluated the efficacy of ClO₂, PAA, sodium hypochlorite (NaOCl), and quaternary ammonium compounds (QACs) against the B. cereus isolate exhibiting the greatest biofilm-forming ability.
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Biofilm-associated cells demonstrated greater disinfectant tolerance than planktonic cells, particularly on polypropylene surfaces. Under conditions without added organic residue contamination, ClO₂ at concentrations of 150–200 parts per million (ppm) reduced biofilm-associated cells to non-detectable levels on both stainless steel and polypropylene surfaces within one minute. PAA and NaOCl generally required longer contact times, while QACs exhibited the lowest antibiofilm efficacy.
When the researchers introduced simulated bakery product residues, disinfectant efficacy decreased. At 5 percent organic contamination, ClO₂ at 100 ppm reduced biofilm-associated cells to non-detectable levels on both surface materials within five minutes. In comparison, PAA at 150 ppm required 10 minutes on stainless steel and 15 minutes on polypropylene.
At 25 percent organic contamination, both disinfectants required 15 minutes to achieve non-detectable levels.
The researchers also observed greater biofilm persistence on polypropylene surfaces, suggesting that plastic food-contact materials may require more intensive cleaning and longer disinfectant contact times.
Findings Highlight Importance of Pre-Cleaning and Surface-Specific Sanitation
The findings emphasized the importance of removing organic residues before disinfection, selecting appropriate disinfectants, and ensuring sufficient contact times to control B. cereus in bakery processing environments.
The researchers noted that post-baking operations, including decorating and packaging, are particularly vulnerable to recontamination because products may contact contaminated surfaces without undergoing subsequent heat treatment.
However, the study's disinfectant experiments were limited to one representative B. cereus isolate under laboratory-simulated conditions. Additionally, the researchers did not evaluate the toxigenic potential of the recovered isolates, meaning their associated pathogenic risk could not be fully assessed.
The researchers recommended further investigation of mixed-species biofilms under industrial processing conditions, as well as studies evaluating toxin-associated genes and toxin production in bakery-derived B. cereus isolates.
