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Neisseria is a genus of bacteria, best known for the pathogenic Neisseria gonorrhoeae and Neisseria meningitidis. Many species of Neisseria are thought to be non-pathogenic or commensal and have a preference for the mucosal surfaces of the human host, especially the oropharynx. Neisseria sicca is a Gram-negative diplococcus long thought to be a common commensal organism of the human upper respiratory tract. N. sicca has often been dismissed by clinical microbiologists as simply a contaminant or colonizer of no clinical significance. As clinical case reports accumulate, an increasing awareness is developing about the dual role of this organism as an opportunistic pathogen. It can be an opportunistic pathogen that causes serious invasive disease, including endocarditis, meningitis and osteomyelitis, when the right conditions are present, especially an immunocompromised host.
Browse more popular Neisseria products: N. gonorrhoeae、N. meningitidis
As with all Gram-negative bacteria, the cell envelope of N. sicca is a complex, multilayered structure. From the cytoplasmic membrane outward, the cell envelope is composed of a thin layer of peptidoglycan (situated in the periplasmic space) and an asymmetric outer membrane. The outer membrane is the outermost barrier between the Gram-negative bacterium and the environment. The outer leaflet of the outer membrane is composed mostly of lipopolysaccharide (LPS), while the inner leaflet is composed of phospholipids. LPS, or endotoxin, is an important virulence factor for pathogenic Neisseria species; this molecule can provoke a strong host response to the bacterium. In the commensal N. sicca, it may contribute to opportunistic infection through its immunostimulatory effect. The outer membrane is studded with various outer membrane proteins (OMPs) which have a number of functions, such as pore-forming porins; receptor proteins involved in nutrient uptake and signal transduction, and adhesins, which aid in bacterial attachment and invasion. Surface appendages (pili) are also located on the cell wall and are important for bacterial motility and adherence.
Figure 1. Untreated Neisseria sicca cells
(Source: Dajani AS, et al. 1976)
While the cell wall of pathogenic Neisseria species (such as N. gonorrhoeae) has been extensively studied, research specifically targeting N. sicca is relatively limited, with most literature published in earlier periods.
N. sicca can cause a diverse range of infections affecting nearly all organ systems. Existing literature, primarily in the form of case reports, outlines its broad clinical spectrum:
Virulence factors of N. sicca are not clearly identified, in contrast with the well-characterized pathogenic Neisseria species. N. sicca is likely to cause disease following a change in host immune status (e.g. immunosuppression, disruption of mucosal barriers), rather than through inherently 'aggressive' virulence factors. Biofilm formation and survival, and its ability to colonize medical devices (pacemakers, dialysis catheters) are likely to be the most important characteristics which allow it to cause infections.
Older literature and case reports have generally shown N. sicca to be susceptible to penicillin, ampicillin, cephalosporins, quinolones, tetracyclines and macrolides. Some case reports of osteomyelitis also included the specific minimum inhibitory concentration (MIC) values of the isolates, which confirmed susceptibility to commonly used antibiotics and provided reference for clinical treatment. Despite N. sicca's general susceptibility to β-lactam antibiotics, knowing its susceptibility profile remains important in guiding appropriate therapy.
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