The resident microflora of voided midstream urine of healthy controls: standard versus expanded urine culture protocols
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The workup and interpretation of urine cultures is not always clear-cut, especially for midstream samples contaminated with commensals. Standard urine culture (SUC) protocols are designed in favor of growth of uropathogens at the expense of commensals. In selected clinical situations, however, it is essential to trace fastidious or new uropathogens by expanding the urine culture conditions (EUC). The aim of our study was to map the microflora in midstream urine specimens from healthy controls by means of EUC, in view of the interpretation of bacterial culture results in symptomatic patients. Midstream urine specimens from 101 healthy controls (86 females and 15 males) were examined using both SUC and EUC. Whilst 73 % of samples examined by SUC showed no growth at 103 colony-forming units (CFU)/mL, 91 % of samples examined by EUC grew bacterial species in large numbers (≥104 CFU/mL). Asymptomatic bacteriuria, as defined by the European guidelines for urinalysis, was detected in six samples with both protocols. EUC revealed 98 different species, mostly Lactobacillus, Staphylococcus, Streptococcus, and Corynebacterium. None of the samples grew Staphylococcus saprophyticus, Corynebacterium urealyticum, or Aerococcus urinae. Samples from females contained higher bacterial loads and showed higher bacterial diversity compared to males. Midstream urine of healthy controls contains large communities of living bacteria that comprise a resident microflora, only revealed by EUC. Hence, the use of EUC instead of SUC in a routine setting would result in more sensitive but less specific results, requiring critical interpretation. In our view, EUC should be reserved for limited indications.
KeywordsLactobacillus Asymptomatic Bacteriuria Streptococcus Agalactiae Midstream Urine High Bacterial Load
The authors would like to thank the healthy volunteers for their cooperation and the technicians of the Department of Laboratory Medicine for their technical contributions.
Compliance with ethical standards
No funding was received.
Conflict of interest
The authors have no conflict of interest.
The study was approved by the Ethical Committee of the Ghent University Hospital with Belgian registration number B670201525210.
All included healthy volunteers gave written informed consent.
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