Transmission of Methicillin‐Resistant Staphylococcus aureus to Preterm Infants Through Breast Milk •

Section of Pediatric Infectious Diseases, University of Chicago, 5841 S. Maryland Avenue MC 5065, Chicago, IL 60637-1463, USA.
Infection Control and Hospital Epidemiology (Impact Factor: 4.18). 10/2004; 25(9):778-80. DOI: 10.1086/502476
Source: PubMed


To determine a potential source of MRSA colonization and infection among preterm infants in a neonatal intensive care unit (NICU) using molecular analysis of breast milk samples.
Case report, outbreak investigation.
Preterm triplets were delivered at 26 weeks' gestation via cesarean section when routine active surveillance for MRSA was performed for all infants in a NICU. Surveillance consisted of swabbing the throat, nose, and umbilicus (TNU) weekly. Although infants A and B initially had negative TNU swabs, repeat cultures were positive for MRSA on day of life (DOL) 10 and DOL 18, respectively. Surveillance and clinical cultures for infant C were negative. Infant A developed sepsis, and multiple blood cultures were positive for MRSA beginning on DOL 14. Infant B developed conjunctivitis and a conjunctival exudate culture was positive for MRSA on DOL 70. Both infants were fed breast milk via nasogastric tube. Cultures of breast milk samples for infants A and B dated prior to either infant's first positive surveillance culture were positive for MRSA. All MRSA isolates had identical results on antibiotic susceptibility testing. PFGE demonstrated identical banding patterns for the MRSA isolates from the blood culture of infant A, breast milk for infants A and B, and a surveillance swab from infant B. At no time did the mother develop evidence of mastitis or other local breast infection.
MRSA can be passed from mother to preterm infant through contaminated breast milk, even in the absence of maternal infection. Colonization and clinical disease can result.

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    ABSTRACT: The chemical composition of breast milk has been studied in detail in the past decades. Hundreds of new antibacterial and antiviral components have been found. Several molecules have been found to promote the proper function of neonatal intestine. However, microbiological studies of breast milk have been, until recently, focused mainly on detecting harmful and pathogenic bacteria and viruses. Natural microbial diversity of human milk has not been widely studied before the work reported in this thesis. This is mainly because breast milk has traditionally been thought to be sterile - even if a certain amount of commensal bacteria have usually been detected in milk samples. The first part of this licentiate thesis contains a short literature review about the anatomy and physiology of breast feeding, human milk chemical and microbiological composition, mastitis, intestinal flora and bacteriocins. The second part reports on the experiments of the licentiate work, concentrating on the microbial diversity in the milk of healthy breast-feeding mothers, and the ability of these bacteria to produce antibacterial substances against pathogenic bacteria. The results indicate that human milk is a source of commensal bacteria for infant intestine. 509 random isolates from 40 breast milk samples were isolated and identified by 16S rRNA sequencing. Median bacterial count was about 600 colony forming units per milliliter. Over half of the isolates were staphylococci, and almost one third streptococci. The most common species were skin bacteria Staphylococcus epidermidis and oral bacteria Streptococcus salivarius and Streptococcus mitis. Lactic acid bacteria, identified as members of Lactobacillus-, Lactococcus- and Leuconostoc -genera, were found in five milk samples. Enterococci were found in three samples. A novel finding in this study is the capability of these commensal bacteria to inhibit the growth of pathogens. In 90 precent of the milk samples commensal bacteria inhibiting the growth of Staphylococcus aureus were found. In 40 precent of samples the colonies could block the growth completely. One fifth of the isolated Staph. epidermidis strains, half of Str. salivarius strains, and all lactic acid bacteria and enterococci could inhibit or block the growth of Staph. aureus. In further study also Listeria innocua- and Micrococcus luteus active isolates were found in 33 and 11 precent of milk samples (out of 140). Furthermore, two Lactococcus lactis isolates from the breast milk were shown to produce bacteriocin nisin, which is an antimicrobial molecule used as a food preservative. The importance of these human milk commensal bacteria in the development of newborn intestinal flora and immune system, as well as in preventing maternal breast infections, should be further explored. Äidinmaidon kemiallista koostumusta on viime vuosikymmeninä tutkittu innokkaasti. Siitä on löydetty satoja uusia yhdisteitä, jotka tuhoavat bakteereita ja viruksia tai edistävät vauvan suoliston toimintaa. Mikrobiologinen tutkimus sen sijaan on rajoittunut yksinomaan haitallisten ja tautia aiheuttavien bakteerien ja virusten seulomiseen. Ihmismaidon luontaista bakteerikoostumusta ei ole ennen tätä tutkimusta juurikaan selvitetty. Osittain tämä johtuu pitkään vallalla olleesta käsityksestä, jonka mukaan äidinmaito on steriiliä. Vaikka maitonäytteissä on hyvin yleisesti, ja toisinaan erittäin runsaasti, harmittomia normaalibakteereita, niitä ei ole pidetty kiinnostavina. Tämän lisensiaattityön ensimmäinen osa sisältää kirjallisuuskatsauksen, jossa käsitellään imetyksen anatomiaa ja fysiologiaa, äidinmaidon kemiallista ja mikrobiologista koostumusta, rintatulehdusta, suolistobakteeriston kehittymistä ja bakteriosiineja. Toisessa osassa kerrotaan tulokset lisensiaattityön kokeellisesta osasta, jossa selvitettiin terveiden äitien rintamaidon normaalibakteeriston lajikoostumusta, sekä niiden kykyä tuottaa antimikrobisia yhdisteitä taudinaiheuttajia vastaan. Kokeellisen osan tulokset osoittavat, että äidinmaidon mukana vauva saa merkittävän määrän normaalibakteereita. Mediaani-bakteeripitoisuus maitonäytteissä oli noin 600 pesäkettä muodostavaa yksikköä millilitrassa. Satunnaisotannalla 40:stä äidinmaitonäytteestä eristetyistä 509:stä anaerobisesti kasvaneesta bakteeri-isolaatista yli puolet oli stafylokokkeja, ja lähes kolmannes streptokokkeja. Yleisimmät lajit olivat tyypillinen ihobakteeri Staphylococcus epidermidis sekä suubakteerit Streptococcus salivarius ja Streptococcus mitis. Viidestä näytteestä löytyi maitohappobakteereita, Lactobacillus-, Lactococcus- ja Leuconostoc-sukuisia lajeja, sekä kolmesta näytteestä enterokokkeja. Keskeisin uusin löydös tässä tutkimuksessa on, että äidinmaidossa esiintyy yleisesti Staphylococcus aureus-, Listeria innocua- ja Micrococcus luteus -bakteerien kasvua estäviä normaalibakteereita. Staph. aureus on yleisin rintatulehdusbakteeri, joka voi aiheuttaa vakavia tulehduksia vastasyntyneillä. Noin 90 prosentissa näytteistä esiintyi Staph. aureus -bakteerin kasvua hillitseviä, ja noin 40 prosentissa selvästi estäviä pesäkkeitä. Äidinmaidosta eristetyistä Staph. epidermidis -kannoista viidennes ja Str. salivarius -kannoista puolet, sekä maitohappobakteereista ja enterokokeista kaikki estivät tai hillitsivät Staph. aureus -bakteerin kasvua. Jatkotutkimuksissa (n = 141) L. innocua-, Staph. aureus- ja M. luteus-bakteerien kasvua estäviä pesäkkeitä löytyi 33, 41 ja 11 prosentista näytteitä. Lisäksi kahden eristetyn Lactococcus lactis -maitohappobakteerikannan osoitettiin tuottavan bakteereita tappavaa nisiini-bakteriosiinia, jota käytetään myös elintarvikkeiden lisäaineena. Tutkimuksen perusteella jatkossa kannattaisi jatkossa ottaa selvää siitä, miten äidinmaidon bakteerit vaikuttavat vauvan suolistomikrobiston ja puolustusjärjestelmän kehittymiseen ja suojaavatko ne mahdollisesti imettävää äitiä rintatulehduksilta.
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