Characterization of radiation-resistant vegetative bacteria in beef

Area of Science:

• Microbiology
• Food Science
• Radiation Biology

Background:

• Ground beef harbors diverse microorganisms, with spoilage bacteria posing significant challenges.
• Food irradiation selectively eliminates microflora, introducing a new factor in microbial dynamics.
• Residual microorganisms after non-sterilizing irradiation doses are predominantly gram-negative coccobacilli.

Purpose of the Study:

• To isolate and characterize radiation-resistant bacteria found in ground beef after irradiation.
• To determine the taxonomic classification and identify the predominant genera of these surviving microorganisms.
• To assess the heat sensitivity and radiation resistance of these isolates and compare them to known resistant forms.

Main Methods:

• Utilized an enrichment procedure for the isolation of radiation-resistant bacteria from irradiated ground beef.
• Employed morphological and biochemical tests for bacterial identification and classification.
• Determined growth temperature ranges, heat sensitivity (D10 values), and radiation resistance (D10 values) of isolates.
• Irradiated a reference culture of Moraxella osloensis for comparative analysis.

Main Results:

• Predominant isolates were identified as Moraxella species based on current classifications.
• These bacteria exhibited a wide growth temperature range (2-50°C) and were relatively heat sensitive (D10 ≤ 5.4 min at 70°C).
• Radiation resistance varied significantly (D10 values from 273 to 2,039 krad), with some exceeding the resistance of known spores.
• Irradiation of Moraxella osloensis induced temporary changes in morphology and penicillin sensitivity, with reversion after subculturing.

Conclusions:

• The identified radiation-resistant bacteria are indigenous to ground beef and not artifacts of the irradiation process.
• These Moraxella isolates represent a significant component of the normal microflora resilient to irradiation.
• The ecological or practical significance of these radiation-resistant bacteria in ground beef remains to be determined.