Haemophilus parainfluenzae
Classification
Higher order taxa
Bacteria; Proteobacteria; Gammaproteobacteria; Pasteurellales; Pasteurellaceae; Parainfluenzae
There are five members of this species group: H. parainfluenzae , H. parahaemolyticus, H.paraphrohaemolyticus, H. pittmaniae, and H. sputorum
Description and significance
Haemophilus parainfluenzae is a common member of the microbiota of the human respiratory tract. It is a Gram-negative bacterium and is nonhemolytic, meaning it does not destroy red blood cells (Norskov-Lauritsen, 2014)[11]. An important characteristic of H. parainfluenzae is that it is able to produce its own heme, a molecule that is needed to create hemoglobin which binds oxygen in the blood. Due to this, H. parainfluenzae can be grown in a lab without having to add an external heme group (Norskov-Lauritsen, 2014)[11]. This ability is also one of the key differences between H. parainfluenzae and the better known Haemophilus influenzae (Norskov-Lauritsen, 2014)[11].
While the pathogenicity of H. parainfluenzae is still debated, it has been linked with many illnesses including endocarditis, chronic obstructive pulmonary disease (COPD), and in rare cases, brain abscesses (Norskov-Lauritsen, 2014; Mitchell and Hill, 2000; Simberkoff, 2012)[11][9][17]. It has been found to be especially pathogenic in immunocompromised patients (Kosikowska et. al., 2016)[6]. Further research is being done on H. parainfluenzae that is uncovering more information on its specific effects and abilities, including antibiotic resistant, proinflammatory responses, genetic transformation, and more (Gromkova, 1998; Middleton, 2003)[5][10].
Genome structure
Certain parts of the genome of H. parainfluenzae has been sequenced and published. The genome sequence of a COPD patient was reported with a total number of paired-end reads of 7,039,708. Its assembly resulted in 187 contigs with a total length of 2,127,067, an average coverage of 439×, and a GC content of 38.9% (Fluit et al., 2020).
H. parainfluenzae is a naturally transformable species, as many of its non-essential genes were attained via horizontal gene transfer (Privitera et al., 1998). Injected transposable elements have been shown to efficiently transpose into many different sites in the genome of H. parainfluenzae (Kuac and Goodgal, 1989). Transposon-like elements located in the plasmid DNA of H. parainfluenzae are connected to antibiotic resistance, and resistance to quinolone has been associated with mutations in gyrA and parC genes (Kuac and Goodgal, 1989; Law et al., 2010).
Cell structure
H. parainfluenzae is a Gram-negative coccobacillus (Fluit et al., 2020). H. parainfluenzae’s cell membrane is mainly composed of phospholipids and cytochromes that both play a role in electron transport systems across the membrane (White and Tucker, 1969). One of the major membrane phospholipids found in H. parainfluenzae is phosphatidylglycerol (White and Tucker, 1969). Located on the cell surface, lipopolysaccharides and lipooligosaccharides are key factors in tissue colonization, adherence to epithelial cells, and persistence of H. parainfluenzae in human hosts (Young and Hood, 2012; Pollard et al., 2008).
Metabolic processes
One of the metabolic characteristics that distinguishes H. parainfluenzae from closely-related H. influenzae is that H. influenzae requires both nicotinamide adenine dinucleotide (NAD) and haemin for growth, whereas H. parainfluenzae only requires NAD (Young and Hood, 2012). Phospholipid metabolism in H. parainfluenzae, specifically metabolism of cardiolipin (CL), occurs at very high rates, despite a relatively low abundance of CL in the cell (Ono and White, 1971). Inhibition of CL metabolism or CL synthesis results in a corresponding inhibition in cell growth, linking CL synthesis and metabolism to vital cellular processes (Ono and White, 1971).
Although not unique to H. parainfluenzae, another process displayed by H. parainfluenzae is its ability to form biofilms (Pang and Swords, 2017). H. parainfluenzae forms biofilms in Otitis Media infections, allowing the bacteria to persist in the presence of antibiotics and host immune responses (Pang and Swords, 2017). Recent studies have shown that H. parainfluenzae’s biofilm is mainly composed of DNA and proteins, not carbohydrates (Pang and Swords, 2017).
Ecology
H. parainfluenzae is a common commensal organism found in the normal flora of the human nose and mouth mucosal surfaces. The genus Haemophilus makes up roughly 10% of the respiratory tract microbiome, and H. parainfluenzae is the dominant species, forming multicellular differentiated biofilm communities (Pang and Swords, 2017; Papov et al., 2019). Recent studies categorize this bacteria as an opportunistic pathogen, causing a variety of diseases and infections (Mitchell and Hill, 2000). The oropharynx environment allows easy access to the human respiratory and gastrointestinal systems, which are the main sites of H. parainfluenzae colonization and infection (Simberkoff, 2012).
Pathology
Although H. parainfluenzae can reside within the mucus of the nose and mouth without significant symptoms, there are still many diseases caused by the bacteria. When clearance of mucus inhabited by H. parainfluenzae is obstructed, the bacteria can spread and cause opportunistic infections of the periodontal, endocardial, pulmonary, and bone tissues (Pang and Swords, 2017). These diseases are usually chronic or recurrent due to bacterial persistence within the biofilm communities, which can have high antibiotic resistance (Pang and Swords, 2017).
H. parainfluenzae most commonly causes upper and lower respiratory tract infections, but has been associated with other diseases such as endocarditis, septic arthritis, genital tract infections, osteomyelitis, psoas and retroperitoneal abscesses, hepatic abscesses, cholecystitis, and meningitis (Popov et al. 2019). There have been cases in which severe abdominal pain was traced back to H. parainfluenzae as well (Popov et al. 2019). In 2019, there was a case of COVID-19 with a patient presenting elevated levels of H. parainfluenzae. (De Castro et al., 2019). Studies have shown the human immune response has specific antibodies to target H. parainfluenzae outer membrane proteins (Mitchell and Hill, 2000).
Antibiotic Resistance
Many of the recent studies on H. parainfluenzae focus on its antibiotic resistance. Beta-lactams can target the Haemophilus genus and are first-line defense for many of the diseases and infections this genus causes (Andrzejczuk et al., 2019). Bacteria such as H. parainfluenzae, have begun to develop resistance to these antibiotics, due to their widespread usage (Andrzejczuk et al., 2019). Studies have shown H. parainfluenzae can express multiple resistance mechanisms, that work alone or together, to confer decreased susceptibility to a variety of beta-lactam antibiotics (García-Cobos et al., 2013).
Current Research
Research is being conducted on Haemophilus parainfluenzae to learn more about the causes and effects of infection. For example, since H. parainfluenzae is associated with various opportunistic infections, significant research has been done on Infective Endocarditis (IE) and Otitis Media (OM), two of the most commonly caused by the microorganism. In a study conducted by Pang and Swords (2017), the authors analyzed H. parainfluenzae biofilms formed during OM infection in vivo. They observed that H. parainfluenzae formed biofilm communities that included a neutrophil extracellular trap, which is a method of protection against infection, and bacteria persisted in the biofilm community (Pang and Swords, 2017).
Similarly, Gromkova et al. (1998) analyzed the ability of H. parainfluenzae isolates to undergo genetic transformation. Previous studies have shown that extracellular release of DNA is common in H. parainfluenzae (Pang and Swords 2017), but this was one of the first studies to analyze the microbe’s transformability. The authors studied biotypes I, II, and III; they found that 13% of biotype I were transformable, 50% of biotype II were transformable, and none of biotype III were transformable. The lack of transformability in certain isolates was due to the cells’ inability to gain competence, not due to poor donor activity of DNA. They also observed that none of the clumping strains were able to undergo transformation, but half of the non-clumping isolates were able to undergo transformation, suggesting that clumping can interfere with transformation (Gromkova et al., 1998). The majority of ampicillin-resistant strains were biotype II, while none of biotype III displayed ampicillin resistance, suggesting a correlation between transformability and antibiotic resistance . More recent research on infective endocarditis has been performed on a difficult clinical case of a pregnant patient who presented with a 104 degree fever, severe headaches, and fatigue. Upon admission to the hospital, the patient tested positive for coronavirus (COVID-19). However, she developed worsening neurological symptoms. After ordering blood cultures, Haemophilus parainfluenzae was present in her blood. They ordered an echocardiogram, where they determined the bacteria was built up within her mitral valve. The patient was diagnosed with infective endocarditis (IE), due to H. parainfluenzae infection (DeCastro et al., 2019).
Continuing research in the thoracic cavity, a study by Middleton et al. (2003) looked at the effects of two different strains of Haemophilus parainfluenzae in the respiratory tract to determine if they were pathogenic. The two strains of H. parainfluenzae (Hpi), Hpi 3698 and Hpi 4846, were tested on different tissues to see if they attached to damage on the tissues, caused damage to the cells of the tissues, or increased production of a pro-inflammatory protein, all of which are typical behaviors of pathogenic bacteria. Only Hpi 4846 adhered to tissue damage while Hpi 3698 did not. Also, Hpi 3698 created tissue damage, but Hpi 4846 did not. Both strains increased the amount of the pro-inflammatory protein in the host (Middleton et al., 2003). The applications of Hpi can be studied further in other parts of the respiratory tract. For example, in some cases, Hpi is a pathogen of only the lower respiratory tract when damaged airway defenses halt the clearance of bacteria. Therefore, Hpi 3698 and Hpi 4846 can most prominently be found in human mucosa of the respiratory tract (Middleton et al., 2003).
References
Edited by Madalena Cardoso, Ashley Clark, Elizabeth Dort, and Jane Hilsenrath, students of Jennifer Bhatnagar for [http://www.bu.edu/academics/cas/courses/cas-bi-311/ BI 311 General Microbiology], 2020, Boston University.
