Kingella kingae is a species of gram-negative

Gram-negative

Gram-negative bacteria are bacteria that do not retain crystal violet dye in the Gram staining protocol. In a Gram stain test, a counterstain is added after the crystal violet, coloring all Gram-negative bacteria with a red or pink color...

 aerobic coccobacilli. First isolated in 1960, it was not until the 1990s that culture techniques improved enough for it to become recognized as a significant cause of infection in young children. It is best known as a cause of septic arthritis

Septic arthritis

Septic arthritis is the purulent invasion of a joint by an infectious agent which produces arthritis. People with artificial joints are more at risk than the general population but have slightly different symptoms, are infected with different organisms and require different treatment. Septic...

, osteomyelitis

Osteomyelitis

Osteomyelitis simply means an infection of the bone or bone marrow...

, spondylodiscitis

Discitis

Discitis or diskitis is an infection in the intervertebral disc space that affects different age groups, but usually spontaneously affects children under 8 years of age. Nonetheless, discitis occurs post surgically in approximately 1-2 percent of patients status post spine surgery.-Signs and...

, bacteraemia, and endocarditis

Endocarditis

Endocarditis is an inflammation of the inner layer of the heart, the endocardium. It usually involves the heart valves . Other structures that may be involved include the interventricular septum, the chordae tendineae, the mural endocardium, or even on intracardiac devices...

, and less frequently lower respiratory tract infections and meningitis.

K. kingae is part of the bacterial flora of the throat

Pharynx

The human pharynx is the part of the throat situated immediately posterior to the mouth and nasal cavity, and anterior to the esophagus and larynx. The human pharynx is conventionally divided into three sections: the nasopharynx , the oropharynx , and the laryngopharynx...

 in young children and is transmitted from child-to-child. When it causes disease, the clinical presentation is often subtle and preceded by a recent history of stomatitis

Stomatitis

Stomatitis is an inflammation of the mucous lining of any of the structures in the mouth, which may involve the cheeks, gums, tongue, lips, throat, and roof or floor of the mouth...

 or upper respiratory infection.

One notable exception is in cases of endocarditis (heart valve infection), which can be more refractory to treatment. K. kingae is the fifth member of the HACEK group

HACEK organism

A HACEK organism is one of a set of slow-growing Gram negative bacteria that form a normal part of the human flora.They are a frequent cause of endocarditis in children.-Organisms:The name is formed from their initials:...

 of fastidious gram-negative bacteria that cause endocarditis. Routine laboratory tests may be normal because the organism is difficult to culture. Inoculating the fluid from infected joints directly into blood culture vials can enhance the chances of an accurate culture, but extended culture times are not helpful.

The organism has also been known as Moraxella kingae.

K. kingae is oxidase-positive

Oxidase test

The oxidase test is a test used in microbiology to determine if a bacterium produces certain cytochrome c oxidases. It uses disks impregnated with a reagent such as N,N,N′,N′-tetramethyl-p-phenylenediamine or N,N-Dimethyl-p-phenylenediamine , which is also a redox indicator...

, catalase-negative

Catalase

Catalase is a common enzyme found in nearly all living organisms that are exposed to oxygen, where it catalyzes the decomposition of hydrogen peroxide to water and oxygen...

, and beta-hemolytic.

Mechanism of Infection

Kingella kingae is thought to begin infection by colonizing the pharynx, crossing the epithelium by using an RTX toxin

RTX toxin

The RTX toxin family is a group of exotoxins produced by gram-negative bacteria. All RTX toxins share a common gene organization and structural appearance. Gram-negative bacteria secrete the toxins into the extracellular space using the type I secretion system. RTX is an abbreviation of "repeats...

, and entering the circulation and reaching deeper tissues, such as bones and joints.

K. kingae express type IV pili

Pili

Pili may refer to:Common names:* Pilus is a cellular organelle* Pili, a Philippine Tree that is a source of the Pili nut* Pili is a Hawaiian grass used to thatch structuresProper names:...

 which allow for enhanced adhesion to respiratory epithelial and synovial cells and thus increased likelihood of colonization. These pili have also been shown to be reduced in number as pathogenesis progresses. σ54 regulates the transcription of pilA1, a major pilus subunit. PilS and PilR, regulatory transcription factors best known from the Pseudomonas aeruginosa

Pseudomonas aeruginosa

Pseudomonas aeruginosa is a common bacterium that can cause disease in animals, including humans. It is found in soil, water, skin flora, and most man-made environments throughout the world. It thrives not only in normal atmospheres, but also in hypoxic atmospheres, and has, thus, colonized many...

pilus system, also may regulate pilA expression. High levels of type IV pili on K.kingae are associated with spreading/corroding colony types, while low levels of type IV pili are associated with nonspreading/non-corroding colony types of K.kingae. There are, in fact, three different types of populations: spreading/corroding (with high-density pilation, nonspreading/noncorroding colonies (low density pilation), and domed colonies (no pilation, and thus no adherence to epithelium). Generally, respiratory and nonendocarditis infections tend to be highly piliated, while joint fluid, bone, and endocarditis blood isolates are less piliated, if at all.

Spondylodiscitis

Children under three years of age may become infected with K.kingae and develop Spondylodiscitis. Typical Symptoms include back pain, abdominal pain, and damage to the bones and joints. It generally targets the lumbar region of the spinal cord, and the only true way of diagnosing it is through biopsy or needle aspiration, as blood plate growth gives many false negatives.

Osteomyelitis

Osteomyelitis occurs in previously healthy children. The infection rate is poorly documented, thus the illness tends to go underdiagnosed. K. kingae can be transmitted person to person in rare cases.
Diagnostic tools include low-grade fever, elevated inflammatory markers (ESR and CRP), and / but white blood cell counts are generally unreliable since they vary among infected patients.
K. kingae infections are generally comcomitant with upper respiratory diseases or stomatitis, since disrupted respiratory or buccal mucosa is likely to facilitate bacterial invasion and hematogenous dissemination.

The cause of osteoarticular infections is frequently not identified. Less than 15% of K. kingae-positive clinical specimens reveal organisms on Gram stain. Risk of misidentification to other more commonly diagnosed bacterium. Fortunately, K. kingae readily treatable with a wide variety of antibiotics such as beta-lactams, tetracyclines, erythromycin

Erythromycin

Erythromycin is a macrolide antibiotic that has an antimicrobial spectrum similar to or slightly wider than that of penicillin, and is often used for people who have an allergy to penicillins. For respiratory tract infections, it has better coverage of atypical organisms, including mycoplasma and...

, and fluoroquinolones . It shows low mininum inhibitory concentration (MIC) for penicillins, rifampicin

Rifampicin

Rifampicin or rifampin is a bactericidal antibiotic drug of the rifamycin group. It is a semisynthetic compound derived from Amycolatopsis rifamycinica ...

, and azithromycin

Azithromycin

Azithromycin is an azalide, a subclass of macrolide antibiotics. Azithromycin is one of the world's best-selling antibiotics...

.
Multiple studies have shown that inoculating synovial fluid or bone samples directly into blood culture bottles substantially increases the detection of K kingae compared with direct plating of specimens on solid media.

As an oropharyngeal colonizer, K kingae is transmitted respiratory secretions, saliva, and potentially oral contact with contaminated objects.
Recent studies suggest K. kingae strains may demonstrate varying degrees of pathogenicity, which could support the person-to-person transfer of pathogenic K. kingae.

Infection has been shown to have high prevalence in the autumn and winter months.

Adult Infections

K.kingae infections are quite rare in adults, but they do occur in immunocompromised patients. Poor oral hygiene, pharyngitis, and mucosal ulceration are also predisposing factors for infection. The infection can occur in the respiratory or urinary tract, as it is a part of the normal flora in those two areas, and will develop into septicemia or septic arthritis.