Cyclic lipodepsipeptides produced by Pseudomonas spp. naturally present in raw milk induce inhibitory effects on microbiological inhibitor assays for antibiotic residue screening.
Two Pseudomonas strains, identified as closely related to Pseudomonas tolaasii, were isolated from milk of a farm with frequent false-positive Delvotest results for screening putative antibiotic residues in raw milk executed as part of the regulatory quality programme. Growth at 5 to 7°C of these isolates in milk resulted in high lipolysis and the production of bacterial inhibitors. The two main bacterial inhibitors have a molecular weight of 1168.7 and 1140.7 Da respectively, are heat-tolerant and inhibit Geobacillus stearothermophilus var. calidolactis, the test strain of most of the commercially available microbiological inhibitor tests for screening of antibiotic residues in milk. Furthermore, these bacterial inhibitors show antimicrobial activity against Staphylococcus aureus, Bacillus cereus and B. subtilis and also interfere negatively with yoghurt production. Following their isolation and purification with RP-HPLC, the inhibitors were identified by NMR analysis as cyclic lipodepsipeptides of the viscosin group. Our findings bring to light a new challenge for quality control in the dairy industry. By prolonging the refrigerated storage of raw milk, the keeping quality of milk is influenced by growth and metabolic activities of psychrotrophic bacteria such as pseudomonads. Besides an increased risk of possible spoilage of long shelf-life milk, the production at low temperature of natural bacterial inhibitors may also result in false-positive results for antibiotic residue screening tests based on microbial inhibitor assays thus leading to undue production loss.
Institute for Agricultural and Fisheries Research (ILVO), Technology and Food Science Unit, Melle, Belgium.
Wim
W
Reybroeck
Ghent University (UGent), Department of Organic Chemistry, NMR and Structure Analysis Unit, Gent, Belgium; Ghent University (UGent), Department of Organic Chemistry, Organic and Biomimetic Chemistry Research Unit, Gent, Belgium.
Matthias
M
De Vleeschouwer
Institute for Agricultural and Fisheries Research (ILVO), Technology and Food Science Unit, Melle, Belgium.
Sophie
S
Marchand
Ghent University (UGent), Department of Organic Chemistry, NMR and Structure Analysis Unit, Gent, Belgium.
Davy
D
Sinnaeve
Ghent University (UGent), Department of Biochemistry and Microbiology, Laboratory of Microbiology, Gent, Belgium.
Kim
K
Heylen
Institute for Agricultural and Fisheries Research (ILVO), Technology and Food Science Unit, Melle, Belgium.
Jan
J
De Block
Ghent University (UGent), Department of Organic Chemistry, Organic and Biomimetic Chemistry Research Unit, Gent, Belgium.
Annemieke
A
Madder
Ghent University (UGent), Department of Organic Chemistry, NMR and Structure Analysis Unit, Gent, Belgium.
José C
JC
Martins
Institute for Agricultural and Fisheries Research (ILVO), Technology and Food Science Unit, Melle, Belgium; Ghent University (UGent), Department of Pathology, Bacteriology and Poultry Diseases, Merelbeke, Belgium.
Marc
M
Heyndrickx
D000818:Animals; D002853:Chromatography, Liquid; D015169:Colony Count, Microbial; D004350:Drug Residues / Q000032:analysis*; D013058:Mass Spectrometry; D008826:Microbial Sensitivity Tests; D008892:Milk / Q000382:microbiology*; D010456:Peptides, Cyclic / Q000096:biosynthesis / Q000302:isolation & purification / Q000494:pharmacology*; D011549:Pseudomonas / Q000378:metabolism*
D010456:Peptides, Cyclic