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Justin Baynham, Allison Moorman, Catherine Donnellan, Vicky Cevallos, Jeremy Keenan; Demographics and Bacterial Contamination Influences in Antibacterial Effectiveness of Human Milk. Invest. Ophthalmol. Vis. Sci. 2013;54(15):2890.
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© ARVO (1962-2015); The Authors (2016-present)
Human milk is used for ocular surface disease in developing countries. A study by our group using an agar well diffusion assay found that human milk has significant antibacterial effect against Neisseria gonorrhoeae, Moraxella catarrhalis, and Streptococcus viridans. One explanation of this effect is competitive inhibition by commensal bacteria. The purpose of this study is to investigate whether maternal or infant age and bacterial contamination in the samples are associated with the antibacterial effect of human milk.
We used an agar well diffusion assay to determine the susceptibility of various bacteria to human milk. For each milk sample, we also measured the amount of bacterial contamination in the specimen by inoculating dilutions of 20 μL of human milk onto blood agar, incubating the plates for 18-24 hours at 35° C in 5% CO2, and counting the number of colony forming units per mL. We assessed for associations between bacterial inhibition and (1) infant age, (2) maternal age, and (3) amount of bacterial contamination using the Spearman’s correlation coefficient.
Twenty-three samples were tested. Maternal age ranged from 22 to 43 years, median 32 years. Infant age ranged from 0.58 to 36 months, median 4 months. The association between infant age and bacterial inhibition was variable: M. catarrhalis (Spearman’s ρ=0.52, p-value=0.01), N. gonorrhoeae (ρ=0.09, p-value=0.70), and S. viridans (ρ=0.19, p-value=0.39). The association between maternal age and inhibition was also dependent on the organism: N. gonorrhoeae (ρ=-0.48, p-value=0.03), M. catarrhalis (ρ=-0.15, p-value=0.53), and S. viridans (ρ=-0.08, p-value=0.72). The results for amount of bacterial contamination and inhibition were: N. gonorrhoeae (ρ=-0.42, p-value=0.05), M. catarrhalis (ρ=0.07, p-value=0.76), and S. viridans (ρ=-0.13, p-value=0.56).
Statistically significant associations were observed for infant age and inhibition of M. catarrhalis and maternal age and inhibition of N. gonorrhoeae. It is difficult to generalize these findings because the results were inconsistent between bacteria. The lack of statistical significance between bacterial contamination and inhibition may indicate that the antibacterial effect of human milk is independent of the load of commensal bacterial. Further study is needed.
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