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Isolation and Identification of Ropy Bacteria in Raw Milk¹

Food Science Program, Department of Animal Sciences, University of Maryland, College Park 20742

ABSTRACT

Approximately 4.2% of 4,000 Maryland-Virginia raw milk tanker samples developed ropiness when incubated at 10C. Of the 56 bacterial isolates 30 were identified by species. Klebsiella oxytoca and Pseudomonas aeruginosa were isolated most frequently. Other ropy isolates were identified as Pseudomonas spp., Chromobacterium, Flavobacterium multivorum, presumptive Yersinia pestis, Enterobacter agglomerans, Klebsiella pneumoniae, and Pasteurella-Actinobacter spp. Six of the Klebsiella oxytoca isolates were mesophilic (optimum temperatures of 32.0 to 37.8°C) with two isolates having psychrotrophic tendencies (optimum temperature of 26.8°C). All Pseudomonas aeruginosa isolates appeared to be psychrotrophic in their temperature requirements (optimum temperatures of 23.0 to 31.0°C).

Klebsiella oxytoca was significant in preliminary development of the ropy condition. All Klebsiella oxytoca isolates developed ropiness within 24 h. The Pseudomonas spp. and Klebsiella pneumoniae isolates required at long as 7 days to develop detectable ropiness at 10°C. A recommended Klebsiella oxytoca differentiation agar is presented as a rapid screening method during outbreaks where Klebsiella oxytoca is the organism of significance.

¹ Scientific Article Number A-3319, Contribution Number 6391 of the Maryland Agricultural Experiment Station, Department of Animal Sciences, Food Science Program, College Park, 20742.