Remember Koch’s Postulates? These are the four criteria designed to establish a causal relationship between a causative microbe and a disease:
- The microorganism must be found in abundance in all organisms suffering from the disease, but should not be found in healthy organisms.
- The microorganism must be isolated from a diseased organism and grown in pure culture.
- The cultured microorganism should cause disease when introduced into a healthy organism.
- The microorganism must be reisolated from the inoculated, diseased experimental host and identified as being identical to the original specific causative agent.
Simply finding a potentially-disease causing organism does not necessarily mean it’s up to no good!
The bacterium Mycoplasma pneumoniae is carried at high rates in the upper respiratory tracts of healthy children and usual diagnostic tests cannot differentiate between such asymptomatic carriage and actual respiratory tract infection, according to a new study. These findings are important as they suggest that clinicians may need to reconsider the clinical significance of a positive test result for the presence of this bacterium.
The researchers compared upper respiratory tract swabs and blood culture results taken from 321 children (aged 3 months to 16 years) admitted to hospital with a respiratory tract infection with those from 405 healthy children undergoing an elective surgical procedure. They found that the prevalence of M. pneumoniae (as measured using PCR tests) did not differ significantly between the asymptomatic group and the symptomatic group. There was also no difference in prevalence when diagnosed using blood tests. In addition, a high rate of other bacteria and viruses was found in both asymptomatic and symptomatic children.
This data indicates that the presence of M. pneumoniae in the upper respiratory tract is common in asymptomatic children. Current diagnostic tests for M. pneumoniae are unable to differentiate between asymptomatic carriage and symptomatic infection. Cinicians may need to readdress the clinical significance of a positive test result.
Carriage of Mycoplasma pneumoniae in the Upper Respiratory Tract of Symptomatic and Asymptomatic Children: An Observational Study. (2013) PLoS Med 10(5): e1001444. doi:10.1371/journal.pmed.1001444
Mycoplasma pneumoniae is thought to be a common cause of respiratory tract infections (RTIs) in children. The diagnosis of M. pneumoniae RTIs currently relies on serological methods and/or the detection of bacterial DNA in the upper respiratory tract (URT). It is conceivable, however, that these diagnostic methods also yield positive results if M. pneumoniae is carried asymptomatically in the URT. Positive results from these tests may therefore not always be indicative of a symptomatic infection. The existence of asymptomatic carriage of M. pneumoniae has not been established. We hypothesized that asymptomatic carriage in children exists and investigated whether colonization and symptomatic infection could be differentiated by current diagnostic methods. This study was conducted at the Erasmus MC–Sophia Children’s Hospital and the after-hours General Practitioners Cooperative in Rotterdam, The Netherlands. Asymptomatic children (n = 405) and children with RTI symptoms (n = 321) aged 3 mo to 16 y were enrolled in a cross-sectional study from July 1, 2008, to November 30, 2011. Clinical data, pharyngeal and nasopharyngeal specimens, and serum samples were collected. The primary objective was to differentiate between colonization and symptomatic infection with M. pneumoniae by current diagnostic methods, especially real-time PCR. M. pneumoniae DNA was detected in 21.2% (95% CI 17.2%–25.2%) of the asymptomatic children and in 16.2% (95% CI 12.2%–20.2%) of the symptomatic children (p=0.11). Neither serology nor quantitative PCR nor culture differentiated asymptomatic carriage from infection. A total of 202 children were tested for the presence of other bacterial and viral pathogens. Two or more pathogens were found in 56% (63/112) of the asymptomatic children and in 55.5% (50/90) of the symptomatic children. Finally, longitudinal sampling showed persistence of M. pneumoniae in the URT for up to 4 mo. Fifteen of the 21 asymptomatic children with M. pneumoniae and 19 of the 22 symptomatic children with M. pneumoniae in this longitudinal follow-up tested negative after 1 mo. Although our study has limitations, such as a single study site and limited sample size, our data indicate that the presence of M. pneumoniae in the URT is common in asymptomatic children. The current diagnostic tests for M. pneumoniae are unable to differentiate between asymptomatic carriage and symptomatic infection.