NON PATHOGENIC NEISSERIA
Other Neisseria spp. exist as normal inhabitants of the upper respiratory tract. Referred to as commensals,
saprophytes, or nonpathogens, these species are occasionally isolated from the genital tract. The commensal
Neisseria spp. rarely cause disease, but they have sporadically been implicated in meningitis, endocarditis, prosthetic valve infections, bacteremia, pneumonia, empyema, bacteriuria, osteomyelitis, and ocular infections (Table 18-7). One must keep in mind that the incidence of infections caused by the commensal Neisseria spp. is extremely low and that the main reason for the microbiologist to be familiar with these organisms is to accurately separate them from the pathogenic species.
Table 18-8 lists the colonial morphology and primary isolation sites of the Neisseria spp. and relate organisms. Table 18-9 lists the traditional and additional tests used to identify the Neisseria spp. And some related genera. These organisms are divided into three groups:
Group 1: Traditional pathogens
Group 2: Commensal Neisseria spp. that may grow on selective medium
Group 3: Commensal Neisseria spp. that do not usually grow on selective medium
Groups 2 and 3 are further divided on the basis of their activities in carbohydrates. Saccharolytic organisms are able to split carbohydrates, whereas asaccharolytic organisms are unable to do so.
In the clinical laboratory, when isolated from respiratory specimens, the commensal Neisseria spp. are usually identified only by Gram stain and gross colony morphology and are called Neisseria spp. or “usual oral flora.” Further identification by biochemical tests is not done. When they are isolated from selective agar medium or sterile body sites, differentiation from the pathogenic Neisseria may be
required. Common laboratory tests and observations do not always adequately differentiate all the commensal species from one another or from the pathogens. In addition, insufficient test parameters and equivocal carbohydrate reactions have led to confusion between the pathogenic and commensal Neisseria spp. Additional tests used to help further differentiate these organisms include growth on nutrient agar at 35° C, growth on SBA or CHOC at 22° C, and the reduction of nitrate and nitrite
Neisseria cinerea was first described in 1906 but was subsequently misclassified as a subtype of M. catarrhalis (Neisseria pseudocatarrhalis). It was called N. cinerea in 1939. The organism has received considerable attention in the past few years because of its misidentification as N. gonorrhoeae in some commercial
identification systems. Although N. cinerea is glucose negative in CTA sugars, in some commercial kits the glucose was read as positive, making it biochemically identical to N. gonorrhoeae.
The colonial morphology of N. cinerea is also similar to the T3 colonies of N. gonorrhoeae (Figure 18-14).
Because of its similarities to N. gonorrhoeae, colistin susceptibility is a helpful test in differentiating the two.
A suspension of the organism is swabbed onto an SBA or CHOC plate, a 10-flg colistin disk is applied to the
inoculum, and the plate is incubated in CO2 for 18 to 24oohours. N. cinerea is susceptible (10 mm or more zone of inhibition) to colistin. Useful tests for differentiation of N. cinerea from M. catarrhalis are reduction of nitrate and negative DNase reaction. Useful observation for differentiation from N. flavescens is lack of yellow pigment production.
Neisseria flavescens (flavescens means “yellow”) is a yellow-pigmented Neisseria spp. that is asaccharolytic (does not use carbohydrates). It can be differentiated from N. cinerea by its ability to grow on SBA or CHOC at 22° C and its yellow colonies.
Neisseria lactamica was reported as early as 1934 but did not become widely recognized as being separate from N. meningitidis until 1968. It is commonly found in the nasopharynx of infants and children and, like N. polysaccharea, is commonly encountered in meningococcal carrier surveys. The carriage rate of this species in children appears to peak at about 2 years of age and to steadily decline from there. It is rarely isolated from adults. It is the only Neisseria spp.
that uses lactose; thus its species designation, lactamica. N. lactamica can be misidentified as N. meningitides because of its similar colony morphology (N. lactamica is a little smaller), its ability to grow on selective
media (e.g., MTM), its carbohydrate reactions (glucose and maltose positive), and some cross-reaction
with meningococcal typing sera (Figure 18-15). The definitive test for differentiation from N. meningitidis
and all other Neisseria spp. is lactose utilization or positive ONPG reaction; however, N. lactamica can exhibit delayed lactose utilization.
Colonies of N. mucosa are large, often adherent to the agar, and very mucoid, giving the species its name. It is usually isolated from the nasopharynx of children or young adults. It has also been isolated from the airways of dolphins. This organism has been documented to cause pneumonia in children. It has the same carbohydrate pattern as N. sicca and N. subflava biovar perf/ava, but it differs from these species in its ability to reduce nitrite to nitrogen gas, its colonial morphology, and its lack of pigment production.
N. polysaccharea was first described by a group of French investigators in 1974 who isolated the organism from throats of healthy children while conducting meningococcal carriage rate surveys. The organism produces large amounts of extracellular polysaccharide when grown in media containing 1%or 5%sucrose; thus the species name. The colonial morphology and carbohydrate utilization (glucose and maltose; rarely sucrose positive) of N. polysaccharea have led to its misidentification as the pathogenic N. meningitidis.
Differential tests to separate N. polysaccharea from N. meningitidis are the ability to grow on nutrient agar at 35° C and production of polysaccharide from 1% or 5% sucrose. Additional differential tests to separate N. polysaccharea from N. subf/ava biovar subf/ava are growth on SBA or CHOC at 22° C and lack of yellow pigment production.
The colonies of N. sicca are usually dry, wrinkled, adherent, and “bread crumb-like” (Figure 18-16). The word sicca in Latin means “dry.” N. sicca and N. subflava biovar perflava are usually the two most common Neisseria spp. found in the respiratory tract of adults.
Differentiation of this organism from N. mucosa and N. subflava biovar perflava has been discussed.
The species name for N. subf/ava means “less yellow’~ (Figure 18-17). It consists of three biovars that differ from one another by their carbohydrate utilization patterns. Differentiation of this species from N. polysaccharea is through its ability to grow on SBA or CHOC at 22° C.
Neisseria elongata and N. weaveri are unique among the members of the Neisseria ~pp., in that they are rods. N. elongata contains three subspecies, elongata, glycolytica, and nitroreducens (formally CDC group M-6).
Tests used to differentiate the three subspecies are catalase, acid production from glucose, and reduction of nitrate. N. elongata subsp. elongata is catalase and nitrate reduction negative but nitrite reduction positive.
N. elongata subsp. glycolytica is catalase positive and a weak acid producer from glucose. N. elongate subsp. nitroreducens is catalase negative, but nitrate and nitrite positive. The catalase test for this group of subspecies is weakly positive or negative compared to the other Neisseria spp. All three subspecies are commensals in the upper respiratory tract and therefore are considered opportunistic pathogens.
Neisseria weaveri is normal oral flora in dogs and can be found in humans in infections following dog bites.
It is named after Robert E. Weaver from the CDC because of his extensive research of this organism. It was previously known as CDC group M5 and is catalase positive; it does not produce acid from any of the carbohydrates traditionally used to identify the Neisseria spp. N. weaveri is a gram-negative rod that does not reduce nitrate but does reduce nitrite to gas.
It is also weakly phenylalanine deaminase positive.
This organism is usually sensitive to penicillin.
The CDC first described K. denitrificans in 1972 and gave it the designation TM-l because of its isolation from throat cultures plated on MTM medium in carrier surveys of N. meningitidis and N. lactamica. In 1976, it was placed in the genus Kingella and given the species name denitrificans because of its ability to reduce nitrate. This organism is normal flora in the upper respiratory tract and rarely causes disease, although it has been associated with endocarditis. Kingella spp.
colonial morphology (if it does not pit the agar) and carbohydrate utilization (only glucose positive) have led to its misidentification as N. gonorrhoeae. The Gram stain is the most definitive differential test to separate these organisms. Although Kingella is a gram-negative rod, at times coccoidal forms may predominate, and the penicillin disk test discussed earlier will reveal its true rod form. Additional differential tests include negative
catalase reaction, ability to grow on agar medium at various temperatures, and reduction of nitrate.