A bizarre series of events in a college microbiology lab class left 33 people—including students, professors, and lab workers—believing they were exposed to a deadly germ. Of those, 32 began taking an antibiotic regimen as a precaution to prevent a life-threatening infection. Two also underwent lumbar punctures—aka spinal taps—after developing mild symptoms.

But none of them had actually been exposed to a dangerous pathogen, a fortunate twist in the strange incident, which otherwise stands to be a cautionary tale for the ages. The incident was published in a recent issue of Morbidity and Mortality Weekly Report.

It began with a simple laboratory exercise. The microbiology lab students were supposed to be given mystery bacteria by their instructors and then identify the microbe based on a series of biochemical tests and morphological characterization.

Unbeknownst to the students, the mystery bacteria was supposed to be Plesiomonas shigelloides, a common, non-scary bacterium. It infects a variety of animals, including fish, and has been known to cause diarrhea in humans. But it can be found throughout the environment and in the gastrointestinal tracts of healthy people, too.

To the lab instructors' great surprise, no one in the microbiology class identified their mystery germ as P. shigelloides. Instead, every single person in the class identified it as Neisseria meningitidis, a dangerous pathogen that can cause meningitis (swelling of the brain and spinal cord) and bloodstream infections, both potentially deadly.

N. meningitidis can spread by aerosols, and in laboratories, it requires specialized handling measures to prevent exposures. That includes respiratory protection and a biosafety cabinet, which is an enclosed, ventilated workspace that prevents the bacteria from spreading in the air. But the students did not have those things; they just had gloves and lab coats.

When the instructors checked the lab class's work, they couldn't find fault with their conclusion. They had all identified the mystery bacterium as a gram-negative, oxidase- and catalase-positive diplococcus with carbohydrate fermentation patterns all consistent with N. meningitidis.

An incident response ensued. University officials found that the stockroom freezer where the P. shigelloides was stored also, for some inexplicable reason, held a vial labeled N. meningitidis.

University officials determined that 33 people linked to the lab class had been exposed. They called in state public health officials, who quickly helped with risk assessments and sample testing, made public health recommendations, and helped with the investigation.

Incident response

The health officials recommended that all the exposed people start taking post-exposure prophylactic antibiotics for 10 days. Of the 33 exposed, all but one person started on antibiotics. Health officials noted that 15 of the 33 (45 percent) had gotten at least one meningococcal vaccination previously. Still, two people developed headaches and sought emergency care; they were given spinal taps to rule out N. meningitidis infection.

Several days later, the state's public health laboratory obtained two samples from the ill-fated lab experiment. They used mass spectrometry and whole-genome sequencing to conclusively identify the mystery bacterium: N. sicca, a nonpathogenic relative of N. meningitidis.

The state lab repeated the carbohydrate fermentation tests that the lab students had used to identify the germ as N. meningitidis. The state lab reproduced their results. They concluded that this odd N. sicca isolate was an "atypical carbohydrate fermenter" that threw off the identification, leading not only to the exposure scare but also to the original misidentification on the vial in the stockroom freezer.

While everyone was safe in the end, the class did experience quite a scare and underwent unnecessary interventions. State and federal health officials conducted a subsequent investigation to figure out how a thought-to-be-dangerous pathogen was stored in the same freezer as harmless bacteria for lab classes and how the two got mixed up.

"The investigation found inadequate inventory controls for identifying, labeling, storing, and segregating biological materials according to handling practices and containment conditions," officials reported. "Additional practice gaps included limited stockroom employee supervision, inadequate availability and use of personal protective equipment, and insufficient documentation of sample inventory, disposition, and chain of custody."

The officials reported that the findings prompted the university to overhaul their procures and protocols.

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