Microbiology of bovine semen and artificial breeding practices under New Zealand conditions

The total number of viable micro‐organisms in semen from 42 bulls used for artificial breeding varied between 20 and 534000/ml, with a mean of 4630/ml. Most were bacteria of the genera Bacillus, Corynebacterium, and Micrococcus; also represented were Aerobacter, Alkaligenes, Escherichia, Flavobacterium, Herellea, Nocardia, Proteus, Sarcina , and Staphylococcus and the yeasts Rhodotorula rubra and Candida guilliermondi . Seminal plasma contained natural inhibitors effective against 80% of the Gram positive and 40% of the Gram negative organisms isolated. There was no correlation between total viable count of semen and concentration of inhibitor in the seminal plasma. The high proportion of organisms in semen which were sen,itive to natural inhibitors and the lack of correlation between total viable count and concentration of inhibitor are consistent with the view that semen bacteria are transient invaders derived from the urethra and preputial cavity. However, no consistent difference was found between first and second ejaculations on the same day. Thirty‐seven strains of bacteria from semen were tested against 13 antibiotics; the predominant bacterial flora was sensitive to a wide range of antibiotics. Pseudomonads, coliforms, and yeasts were among the most resistant kinds of micro‐organisms. It is suggested that bulls be so managed that development of antibiotic resistant microbial populations in the urogenital tract is avoided, and also that additional antibiotics be included in the extender to control contaminants which enter in small numbers and subsequently proliferate. A survey showed that most contaminants in the final extended semen were introduced in the component semen or extender; ancillary equipment and materials contributed few bacteria, but apparatus used to bubble extender with C0 2 and bark corks used to close tubes of dispensed semen ma) have been important sources of yeast contamination. The bacterial population of extended semen stored at 15°c declined to a low level in 3 days, but the yeast population increased and constituted most of the total viable count after 2 days. Nystatin added to extender controlled yeast growth.