Probiotic nose drops reduced the likelihood of students being colonized with the bacteria that cause meningitis, according to a study published online March 25 in Clinical Infectious Diseases.

The nose drops contained Neisseria lactamica, a non-disease-causing relative of the bacterium that causes meningitis, Neisseria meningitidis. Epidemiological studies had previously shown that people who carry N lactamica in their nose and throat are both less likely to be colonized with N meningitidis and less likely to contract meningitis than people who do not, possibly because colonization with one species prevents colonization with the other.
“Neisseria lactamica may therefore be a potential ‘bacterial medicine’ to suppress meningococcal outbreaks,” write Alice M. Deasy, MBBS, from the Department of Infection and Tropical Medicine, Sheffield Teaching Hospitals Foundation Trust, and colleagues. They note that the effect on N meningitidis colonization was stronger and happened more quickly than the effect seen in previous studies from meningococcal vaccine.

The study involved 310 students from two universities in Sheffield, United Kingdom. Each received either an inoculation of 104 colony-forming units of N lactamica or a sham inoculation. The investigators took samples of the students’ oropharyngeal bacteria at 2, 4, 8, 16, and 26 weeks after inoculation.

At baseline, a similar fraction of each group already carried N meningitidis: 24.2% of the treatment group and 22.4% of control patients. (Only a handful of participants in each group [1.9%; 95% confidence interval, 0.4% – 3.5%] carried N lactamica at baseline.) After inoculation, some, but not all, participants in the treatment group became colonized with N lactamica: 33.6% (95% confidence interval, 25.9% – 41.9%) 2 weeks after inoculation, rising to 41.0% (95% confidence interval, 33.0% – 49.3%) by 26 weeks.

Conversely, the proportion of students colonized with N meningitidis dropped after inoculation, going from 24.2% to 6.7% after 4 weeks, and then partially rebounding to 14.5% at 26 weeks. Colonization in the control group was not reduced.

The loss of N meningitidis was greater in the students who were colonized with N lactamica (P = .013), suggesting N lactamica competes with and displaces N meningitidis. These participants were also less likely to acquire new colonization with N meningitidis (P = .011). Of those who were colonized, 13.3% acquired N meningitidis within the 26-week study period compared with 29.2% of control patients and 28% of those who received the inoculation but were not colonized by the probiotic strain.
At the end of the study, all participants received another dose of N lactamica and were tested 2 weeks later. Those who had been in the treatment group were still able to be colonized by N lactamica, and those who had been in the control group showed a similar reduction in N meningitidis carriage as the treatment group had shown at the beginning of the study.

On the basis of these and previous results, including the ability of the treatment group to be recolonized with N lactamica, the investigators write that the protective effect is probably not a result of cross-protective antibodies; rather, “[t]he mechanism is either microbial competition within the nasopharynx, or innate immune responses that operate only in individuals actively carrying the commensal.”