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The use of human milk in the treatment of ocular surface disease is documented in ancient Egyptian, Indian, Greek, Roman and Byzantine texts.1 Galen recommended human milk specifically for conjunctivitis.1 Ocular application of human milk continues today in the developing world, and is recommended in widely distributed parenting guidebooks. Despite these endorsements, the antibacterial activity of topically applied breast milk has not been adequately studied. In this report, we assess the inhibitory effects of human breast milk against common ocular pathogens.
Mothers attending routine outpatient paediatric visits at the University of California, San Francisco, donated milk samples for this study. Subjects with known immunodeficiency, systemic infection or antibiotic use within 2 weeks were excluded. Milk samples were stored at 4°C; the samples were tested within 6 h of collection, or frozen for later testing.
We tested several reference strains of bacteria from the American Type Culture Collection (ATCC): Escherichia coli (ATCC 25922), Haemophilus influenzae (ATCC 49247), Neisseria gonorrhoeae (ATCC 49226), Pseudomonas aeruginosa (ATCC 27853), and Streptococcus pneumonia (ATCC 49619). We also tested patient isolates of Staphylococcus aureus, Moraxella catarrhalis, coagulase-negative Staphylococcus and viridans group Streptococcus. We adapted the Clinical and Laboratory Standards Institute antimicrobial disk susceptibility test standards to suit an agar well diffusion assay.2 ,3 Briefly, we inoculated a 0.5 McFarland turbidity standard suspension onto appropriate susceptibility agar plates. We bored six 8-mm wells into the agar and filled each with 200 μl of either human milk, sterile saline or polymyxin B sulfate/trimethoprim ophthalmic solution (Allergan, Irvine, California, USA). We measured the zone of inhibition with callipers after incubation at 35°C in 5% CO2 for 18–24 h. We chose polymyxin B sulfate/trimethoprim as the positive control since it is a broad-spectrum antibiotic with Gram-positive and Gram-negative coverage. We also inoculated eight milk samples onto blood and chocolate agar plates for speciation of commensal bacteria using standard microbiological methods.
We collected human milk from 23 women (median age 32, IQR 28–37 years). Of the nine bacterial species tested, three were significantly inhibited by human milk relative to the negative control: N gonorrhoeae, M catarrhalis and viridans group Streptococcus (figure 1). Growth of coagulase-negative Staphylococcus was inhibited by several milk samples, but was not statistically significantly different from the control. The antibiotic control resulted in larger zones of inhibition than did human milk for all species except for N gonorrhoeae. The eight milk samples cultured for speciation of commensal bacteria grew a variety of bacteria, mostly skin and gut flora (table 1).
In this study, human milk consistently inhibited N gonorrhoeae growth, with similar potency to polymyxin B sulfate/trimethoprim ophthalmic solution. Human milk had a significant but less pronounced inhibitory effect on M catarrhalis and viridans group Streptococcus, but no inhibitory effect on the other bacteria tested.
Approximately 70% of cases of paediatric conjunctivitis are bacterial, caused most frequently by H influenzae, Streptococcus pneumonia and M catarrhalis.4 The results of this study suggest that human milk is unlikely to be effective against most of the common causes of paediatric conjunctivitis. Moreover, potentially pathogenic bacteria were isolated from several of the milk samples in this study, including S aureus and Enterococcus spp., as well as Gram-negative rods, raising the possibility of superinfection in treated eyes.
The milk samples were particularly effective against N gonorrhoeae, which is a major cause of ophthalmia neonatorum in the developing world.5 This finding was unexpected, and deserves further study, especially given the gravity of ophthalmia neonatorum, and the very real possibility for disruption in prophylactic antibiotic programmes in resource-poor settings.
Contributors JTLB, VC and JDK designed the study. MAM collected the milk specimens. JTLB, CD and VC performed the microbiology experiments. JTLB and JDK analysed the data. JTLB wrote the first draft of the manuscript. JTLB, MAM, CD, VC and JDK critically revised the manuscript. All authors approved of the final manuscript.
Funding This work was supported by a career development award from Research to Prevent Blindness (JDK).
Competing interests None.
Ethics approval University of California, San Francisco Committee on Human Research.
Provenance and peer review Not commissioned; internally peer reviewed.
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