The most common gram-negative cocci of medical importance belong to the family Neisseriaceae which includes the genera Neisseria, Moraxella, Kingella, Acinetobacter, etc.
Gram Negative Cocci
Neisseriaspecies
The genus Neisseriacontains 12 species, two of which, N.meningitidis and N. gonorrhoeae, are commonly pathogenic in humans. Both may cause infections in the CNS, primarily meningitis and meningoencephalitis, but also abscesses, empyemas, and thrombophlebitis.
N.meningitidis
N. meningitidis (Meningococci) is a gram-negative, nonsporeforming diplococcus. It is a leading cause of fatal bacterial meningitis. Meningitis may be accompanied by the appearance of petechiae (rash) associated with meningococcal bacteremia (i.e.,meningococemia). Bacteremia leads to thrombocytopenia, disseminated intravascular coagulation, and shock. Less common infections include conjunctivitis, pneumonia, and sinusitis.
N. gonorrhoeae
It is a urogenital pathogen that is transmitted by sexual contact. Genital infections include purulent urethritis in males and cervicitis in females. Other localized conditions include pharyngitis, anorectal infection, and conjunctivitis (ophthalmia neonatorum). Disseminated gonococcal diseases are pelvic inflammatory disease, bacteremia, arthritis, and metastatic infections at other body sites.
Moraxella (Branhamella) catarrhalis
Moraxella catarrhalis, a gram-negative diplococcus, is an exclusively human pathogen and is a common cause of otitis media in infants and children.  M.catarrhalis causes acute, localized infections such as sinusitis, bronchopneumonia, and life-threatening, systemic diseases including endocarditis and meningitis. M. catarrhalis can cause pneumonia, especially in patients with alcoholism or chronic obstructive lung disease. M. catarrhalis also causes exacerbations of chronic obstructive pulmonary disease (COPD) and chronic bronchitis in elderly patients.
Veillonellaspecies
 They are rarely the sole or predominant organism in sputum smears of patients with pneumonia.
Gram Negative Coccobacilli
A coccobacillus is a type of bacterium with a shape intermediate between cocci and bacilli, i.e., they are very short rods that may be mistaken for cocci. These tiny, pleomorphic gram-negative bacteria range in shape from round (cocci) to short, thin rods (bacilli); hence the bacteria are called “coccobacilli.”
Haemophilus species
Infections most commonly associated with encapsulated strains of Haemophilus influenzae include meningitis, epiglottitis, cellulitis with bacteremia, septic arthritis, ad pneumonia. Â Non-capsulated strains usually cause localized infections such as otitis media, sinusitis, and conjunctivitis.
Haemophilus ducreyi
It causes sexually transmitted infections, chancroid, a major cause of genital ulceration in developing countries characterized by painful sores on the genitalia.
Acinetobacterspecies
****Outbreaks of Acinetobacter infections typically occur in intensive care units (ICUs) and healthcare settings housing very ill patients. Acinetobacter baumannii accounts for about 80% of reported infections. Acinetobactercauses a variety of diseases, ranging from pneumonia to serious blood or wound infections, and the symptoms vary depending on the disease. Acinetobacter may also “colonize” or live in a patient without causing infection or symptoms, especially in tracheostomy sites or open wounds.
Kingellaspecies
Kingella kingae is a species of Gram-negative aerobic coccobacilli. Kingella kingae is recognized increasingly as a cause of skeletal infections (osteomyelitis/septic arthritis ) in children.
Francisellaspecies
Francisella is an intracellular, facultative, gram-negative coccobacillus. Francisella tularensis is the agent of human and animal tularemia.
References
- Bailey & Scott’s Diagnostic Microbiology, Forbes, 11th edition
- Madigan Michael T, Bender, Kelly S, Buckley, Daniel H, Sattley, W. Matthew, & Stahl, David A. (2018). Brock Biology of Microorganisms (15th Edition). Pearson.