Recruitment was paused in March 2020 due to the COVID pandemic. An interim data analysis was conducted in 2021 which confirmed that sufficient data had been collected to meet the primary objective, therefore the study was terminated.

This study is part of a project that aims to develop a vaccine with N. lactamica that prevents meningitis. The investigators have previously given nose drops containing N. lactamica to over 340 volunteers, and shown that many of the volunteers (35-60%) become colonised without causing any illness or disease. In the future the investigators would like to modify N. lactamica so that it can carry vaccine molecules into the nose of children. To do this the investigators need to know more about the immune response generated against N. lactamica. Previously the investigators have shown that inoculation resulted in an immune (antibody) response in volunteers who were colonised. Taking an antibiotic called ciprofloxacin will treat N.lactamica in the nose and throat of the volunteers. The investigators need to know if the immune response to N. lactamica is the same when colonised volunteers are treated with the antibiotic after 4 days, is the same if the investigators treat volunteers after 14 days of carriage. This information will inform future studies.

N. lactamica (Nlac) has been shown to be safe to use in human challenge experiments. Even at a very low dose of 10,000 colony forming units (cfu) long lasting colonisation with Nlac is easily induced in 35-65% of volunteers. The investigators have previously showed that increasing the inoculum to 100,000 cfu increased the subsequent carriage of Nlac to 50%. In 80-90% of those volunteers successfully colonised, this is detectable by 1-2 weeks.

Data regarding earlier detection of colonisation is currently lacking.

Colonisation has a clear effect on the volunteers nasal mucosal microbiome, in that meningococcal acquisition is effectively inhibited in volunteers who carry the organism. Colonisation is immunogenic, with an increase in specific serum IgG by 2 weeks and specific salivary IgA by 4 weeks.

No antibiotic eradication therapy has previously been given following experimental inoculation but Ciprofloxacin has been shown to be effective in the eradication of N. meningitidis.

To design future planned studies using Nlac nasal inoculation and colonisation in order to prevent invasive N. meningitidis disease, it is necessary to further evaluate the colonisation kinetics and efficacy of the eradication following antibiotic treatment. In previous challenges the investigators inoculated volunteers with Nlac and followed those volunteers for prolonged periods of time (over 6 months).

This study will compare the effect of short (4 days) versus longer (14 days) periods of nasal carriage of Nlac in volunteers on immunogenicity, and confirm the efficacy of antibiotic eradication therapy with ciprofloxacin.

Healthy adult volunteers will receive a nasal inoculation of Nlac with an antibiotic given on day 4 or 14. This information will be used to inform the design of future research into the colonisation and immunogenicity of related organisms.

Before designing future protocols the study investigators need to know whether a short containment of the volunteers will be sufficient for immunogenicity, and how quickly the volunteers can be discharged after antibiotic treatment.

A wild-type strain of Nlac (Y92-100) will be used for this study, selected because the investigators have previously used it safely in experimental challenge of over 340 human volunteers.

The same strain will be the parent strain for any future GMO work.

Inclusion Criteria:

• Healthy adults aged 18 to 45 years inclusive on the day of enrolment
• Fully conversant in the English language
• Able and willing (in the investigator's opinion) to comply with all study requirements
• Written informed consent to participate in the trial
• Willingness to take an antibiotic regimen after inoculation according to the study protocol
• For females only, willingness to practice continuous effective contraception (see below) during the study and a negative pregnancy test on the day(s) of screening and inoculation

Exclusion Criteria:

• Current active smokers
• N. lactamica or N. meningitidis detected on throat swab or nasal wash taken before the challenge
• Individuals who have a current infection at the time of inoculation
• Individuals who have been involved in other clinical trials involving receipt of an investigational product over the last 12 weeks or if there is planned use of an investigational product during the study period
• Individuals who have previously been involved in clinical trials investigating meningococcal vaccines or experimental challenge with N. lactamica
• Use of systemic antibiotics within the period 30 days prior to the challenge
• Any confirmed or suspected immunosuppressive or immune-deficient state, including HIV infection; asplenia; recurrent, severe infections and chronic (more than 14 days) immunosuppressant medication within the past 6 months (topical steroids are allowed)
• Use of immunoglobulins or blood products within 3 months prior to enrolment.
• History of allergic disease or reactions likely to be exacerbated by any component of the inoculum
• Contraindications to the use of ciprofloxacin, specifically a history of epilepsy, prolonged QT interval, hypersensitivity to quinolones or a history of tendon disorders related to quinolone use
• Any clinically significant abnormal finding on clinical examination
• Any other significant disease, disorder, or finding which may significantly increase the risk to the volunteer because of participation in the study, affect the ability of the volunteer to participate in the study or impair interpretation of the study data, for example recent surgery to the nasopharynx
• Occupational, household or intimate contact with immunosuppressed persons
• Pregnancy or lactation

From enrolment until the end of follow up - 32 days post inoculation for Group 1 and 42 days post inoculation for Group 2

Adverse events were reported for all enrolled participants for the duration of their involvement in the study and were defined as per the ClinicalTrials.gov definitions