Microbial Degradation of Chlorpyrifos Residues and Their Effects on Calcium Levels In Fresh And Processed Milk in Nakuru County, Kenya
Nyabiba, Asamba Micah
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Acaricides are a class of pesticides used to control mites and ticks. The most widely applied acaricides belong to the organophosphorus (OP) group, including chlorpyrifos (CP) and Diazinon, which have high to moderate toxicity. They accumulate in animal tissues, milk, and other animal products, becoming a threat to human health. The presence of these residues and their metabolites in milk also alters milk composition and chemistry, including vital elements such as calcium. Some microorganisms can use acaricides as a source of energy and carbon, thus serving a critical role in bioremediation. The aim of the study was to determine chlorpyrifos levels in milk and milk products in Nakuru County and to isolate bacteria with CP degradation potential. A pre-study survey showed that dairy farmers in Nakuru County use various acaricides to control ticks, including the organophosphates chlorpyrifos and diazinon. Milk, water, soil, and dipwash samples were collected from selected farms in the county. High-Performance Liquid Chromatography was used to quantify CP residue levels in the collected samples. Approximately 53 % of raw milk samples had significant detectable chlorpyrifos levels, but no residues were detected in processed milk or high-value milk products. All the positive milk samples exceeded the maximum residue limit (MRL). The positivity rate in soil, dip wash, and water samples were 72.7 %, 62.5 %, and 27.3 %, respectively. No significant correlation was found between CP residues and total bacterial colony counts (r = 0.019; p = 0.952) in milk. Calcium levels in the milk samples were within recommended levels. A weak and negative correlation was found between calcium concentration and chlorpyrifos residue levels (r = -0.2, p = 0.636). Using enrichment culture technique, in Minimum Salt Medium (MSM) containing varying concentrations of chlorpyrifos (5 mg/l, 10 mg/l, and 40 mg/l), 18 isolates were found capable of degrading chlorpyrifos and coded MA1 to MA18. The degradation products were detected and identified by gas chromatography-mass spectrometry. There were no significant differences in growth among the three concentrations of chlorpyrifos used. The isolates were subjected to cultural, morphological, biochemical, and molecular characterization. Gene sequence analysis of 16S rRNA showed that the isolates with a potential to degrade and utilize chlorpyrifos as the sole carbon source belonged to the genera Pseudomonas, Stenotrophomonas, Bacillus, Alicaligenes, Lysinibacillus, and Achromobacter. GC-MS analysis showed that the main degradation product was 2-Hydroxy-3,5,6-trichloropyridine (TCP). Significant CP degradation was reported in four isolates: MA1 (87.16 %), MA2 (82.04 %), MA4 (89.53 %%) and MA8 (91.08 %). The findings show that soil in dairy farms in Nakuru County consists of different chlorpyrifos degrading bacteria, which grow in different physical-chemical conditions. The isolated strains can be multiplied and developed for bioremediation and biodegradation of organophosphate-contaminated soil and water.