A microbiology lab exercise at a US college went badly wrong after 33 students, professors and lab workers misidentified a bacterial sample as Neisseria meningitidis, a dangerous pathogen that can cause meningitis and bloodstream infections. The mix-up prompted a full public health response, with 32 of those exposed placed on a 10-day course of preventive antibiotics as a precaution, even though the actual bacterium involved turned out to be harmless. The incident, detailed in a recent Morbidity and Mortality Weekly Report, is being held up as a cautionary tale about laboratory storage and safety procedures.

The students had been asked to identify a supposedly harmless mystery bacterium, Plesiomonas shigelloides, but instead unanimously concluded it was N. meningitidis based on consistent biochemical test results, despite lacking the specialist respiratory protection and biosafety cabinets normally required when handling such a pathogen. An investigation found a vial labelled N. meningitidis had inexplicably been stored in the same freezer as the intended sample. Two people developed headaches and underwent spinal taps as a precaution, and 15 of the 33 had prior meningococcal vaccination. Subsequent genetic testing by state health laboratories revealed the true culprit was N. sicca, a non-pathogenic relative whose unusual fermentation pattern had misled the class's identification.