Isolation and identification of ropy bacteria in raw milk

Approximately 4.2% of 4,000 Maryland-Virginia raw milk tanker samples developed ropiness when incubated at 10 degrees C. 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 degrees C) with two isolates having psychrotrophic tendencies (optimum temperature of 26.8 degrees C). All Pseudomonas aeruginosa isolates appeared to be psychrotropic in their temperature requirements (optimum temperature of 23.0 to 31.0 degrees 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 degrees C. A recommended Klebsiella oxytoca differentiation agar is presented as a rapid screening method during outbreaks where Klebsiella oxytoca is the organism of significance.