Doctor of Philosophy Degree in Pharmacology and Toxicology
Abstract
Staphylococcus aureus (S. aureus) is a ubiquitous Gram-positive bacterium commonly encountered in the environment as well as mucus membranes of animals and humans. S. aureus food poisoning results from consumption of preformed S. aureus enterotoxins in food. The prevalence of antimicrobial resistant (AMR)-S. aureus including methicillin-resistant S. aureus (MRSA) in livestock raw milk consumed by pastoralists in Kenya remains unclear. This study determined the phenotypes, genetic determinants for virulence associated with S. aureus enterotoxins, the phenotypic resistance profiles, genetic determinants responsible for antimicrobial resistance phenotypes of milk borne S. aureus and the relationship between antimicrobial usage (AMU) and emergence of multidrug-resistant (MDR) S. aureus.
A cross-sectional study design involving 188 households in the Marsabit and Isiolo Counties was conducted. In total, 603 milk samples from various livestock including cattle, goats, sheep, camels and 188 pooled from all animals were collected from Isiolo and Marsabit counties of Kenya. S. aureus isolates were cultured from milk samples using a selective media and biochemical tests followed by amplification of S. aureus specific staphylococcal terminase gene (satm) and BLAST analysis.
Antimicrobial susceptibility testing was performed using the Kirby-Bauer disc diffusion method. At least one type of S. aureus enterotoxin gene (SE) was detected in 74.11% (95% CI: 63.91–82.24%) of the 85 isolates. The most frequently encountered gene was see (51; 60%) followed by sea (22; 25.88) and sec (19; 22.35%). None of the isolates tested positive for sed. Overall, 21 of the 85 (24.7%) strains harbored more than one enterotoxin gene.The S. aureus isolates were mainly resistant to tetracycline (79%), ampicillin (58%), and oxacillin (33%), respectively. A few isolates (5–18%) were resistant to clindamycin, cephalexin, erythromycin, kanamycin, and ciprofloxacin. Most of the MDR-S. aureus isolates were MRSA (94%). The genetic determinants found in the AMR isolates included tetK/tetM (96.5%/19%) for tetracycline, blaZ (79%) for penicillin, aac (6’)/aph (2”)/aph (3’)-IIIa (53%) for aminoglycosides, mecA (41%) for oxacillin, and msrA/ermA (24%/7%) for macrolides.
The antimicrobial classes used in the study site were as follows; 4,168 kg of oxytetracycline, 70 kg of sulfonamides, 49.7 kg of aminoglycosides, 46 kg of beta-lactams, 39.4 kg of macrolides, and 0.52 kg for trimethoprim. Oxytetracycline usage was correlated to tetK/tetM (r = 0.62/1) detection, penicillins to mecA/blaZ (r = 0.86/0.98), aminoglycoside to aac (6’)/aph (2”)/aph (3’)-IIIa (r = 0.76/-13), and macrolide usages for detection of ermA/msrA (r = 0.94/0.77).
In conclusion, milk consumed in Marsabit and Isiolo counties is contaminated with enterotoxigenic multidrug resistant S. aureus that harbour antimicrobial resistance genes hence its consumption could increase the risk of staphylococcal food poisoning. Judicious use of antimicrobials is recommended in the study areas to mitigate against development of AMR
