Mannheimia haemolytica

Other Scientific Names

• Bacillus bovisepticus
• Pasteurella haemolytica
• Pasteurella mastitides

• Domain: Bacteria
•     Phylum: Proteobacteria
•         Class: Gammaproteobacteria
•             Order: Pasteurellales
•                 Family: Pasteurellaceae
•                     Genus: Mannheimia
•                         Species: Mannheimia haemolytica

M. haemolytica (from Greek haima - blood, lyt – adverb form of verb lyo - dissolve, and adjectival suffix – ikos latinized in – ica) was formerly classified as Pasteurella haemolytica biogroup 1, and there were two P. haemolytica biotypes, A and T for arabinose or trehalose fermentation (Bisgaard and Mutters, 1986). Seventeen serotypes were described for P. haemolytica. Biotype T strains, i.e. serotypes 3, 4, 10, 11, 15 and 17, were reclassified as Pasteurella trehalosi and later as Bibersteinia trehalosi (Blackall et al., 2007; Sneath and Stevens, 1990). The biotype A strains, i.e. serotypes 1, 2, 5, 6, 7, 8, 9, 12, 13, 14 and 16, were reclassified as M. haemolytica (Angen et al., 1999a; Angen et al., 1999b), which is the subject of this datasheet. The name Mannheimia is a tribute to Walter Mannheim, a German microbiologist who studied the taxonomy of the family Pasteurellaceae (Angen et al., 1999a; Angen et al., 1999b).

M. haemolytica has likely been a commensal within the respiratory tract of cattle for decades. Originally called Bacillus bovisepticus, it was recognized as a pathogen in bovine pneumonia in the early 20^th century (Hepburn, 1925; Jones, 1921). Much of the early understanding of the bacterium in respiratory disease is clouded by it not being readily separated from Pasteurella multocida infections.

(This section includes information from Confer (2009)).

Mannheimia haemolytica is a Gram-negative, non-motile, small rod- and coccobacillus-shaped bacterium. It is often bipolar stained (‘safety pin’) with Gram’s stain. Colonies are smooth and greyish on blood agar and are 1-2 mm in width after 24 h of incubation. Most strains show a characteristic faint β-haemolysis on bovine blood agar. Biochemical reactions are several: D-sorbitol, D-xylose, maltose and dextrin are fermented, while no strains ferment L-arabinose or glucosides. Strains are negative for ornithine decarboxylase and β-glucosidase and positive for α-fucosidase (Angen et al., 1999a).

M. haemolytica was formerly classified as Pasteurella haemolytica biogroup 1, and there were two P. haemolytica biotypes, A and T for arabinose or trehalose fermentation (Bisgaard and Mutters, 1986). Seventeen serotypes were described for P. haemolytica. Biotype T strains were reclassified as Pasteurella trehalosi and later as Bibersteinia trehalosi (Blackall et al., 2007; Sneath and Stevens, 1990). The bacterium M. haemolytica is composed of serotypes 1, 2, 5, 6, 7, 8, 9, 12, 13, 14 and 16 (Angen et al., 1999a; Angen et al., 1999b). The G and C content of the DNA of the type strain is 43.6 molar percentage.

M. haemolytica is part of the normal nasopharyngeal flora in cattle, sheep and goats, and when the animal is stressed or suffers from a viral infection, the bacterium proliferates and is inhaled into the lungs where it can cause severe pneumonia (Rice et al., 2007). Although most of the cases of pneumonia arise from the M. haemolytica strain that is carried by the particular bovine, molecular evidence indicates that there is a certain amount of horizontal transmission from animal to animal during an outbreak of pneumonia (Timsit et al., 2013).

There are many pathogenicity (virulence) factors of M. haemolytica: fimbriae, capsule, lipopolysaccharide (LPS), proteins and lipoproteins of the outer membrane, systems of iron uptake, leukotoxin and extracellular enzymes (Confer, 2009). Each pathogenicity factor is described in detail below.

Capsule

M. haemolytica is encapsulated during the logarithmic growth phase. The capsule is composed of disaccharide repeats of N-acetylmannosaminuronic acid β-1,4 linked with N-acetylmannosamine (McKerral and Lo, 2002). Encapsulation enhances bacterial resistance to phagocytosis and complement mediated killing and functions as an adhesin in the lung (Chae et al., 1990).  The capsular polysaccharide is immunogenic; however, antibody responses to capsular polysaccharide do not correlate with resistance against experimental challenge (Confer, 2009).

Fimbriae

At least two types of fimbriae have been demonstrated, in vitro, on serotype 1, namely a rigid 12 nm-wide structure and a flexible 5 nm-wide fimbria (Morck et al., 1987; Potter et al., 1988). In vivo, strains isolated from broncho-alveolar lavage or adherent to the tracheal epithelium have structures that resemble fimbriae. Their role is not completely known, but they are likely to be one of the adhesins responsible for adhesion of the bacteria to the respiratory epithelium.

Lipopolysaccharide

As in other Gram-negative bacteria, the cell wall of M. haemolytica contains lipopolysaccharide (LPS); the lipid A component is responsible for endotoxin-induced local and systemic pathologic processes. There are variations in LPS profiles among and within serotypes.  For instance, serotype 1 isolates are often smooth  type LPS, whereas serotype 2 isolates are usually rough types (McCluskey et al., 1994). The intravenous injection of purified LPS in calves causes release of thromboxane A₂, prostaglandins, serotonin, cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) (Rice et al., 2007). All or some of these mediators may be responsible for the clinical signs associated with endotoxic shock. If this situation arises, the antibodies directed against LPS of M. haemolytica do not correlate with resistance against experimental infection. Within the respiratory tract, LPS results in vascular damage through release of proinflammatory cytokines, activation of complement and coagulation cascade, and direct cytolysis (Singh et al., 2011).

Proteins and lipoproteins of outer membrane

Many proteins of the outer membrane, with a molecular weight (MW) of 28-40 kDa, can be chemotactic for neutrophils but reduce neutrophilic phagocytosis and lysis of the ingested bacteria (Iovane et al., 1998). Three tandem, membrane lipoproteins (MW=29-30 kDa) act by opposing activation of the complement cascade (Murphy et al., 1998). Two major proteins of the outer membrane, PomA and PomB, have been identified in strains belonging to serovar 1. PomA protein is similar to OmpA of Escherichia coli and has subsequently been referred to as M. haemolytica OmpA; it binds fibronectin and facilitates adhesion to bronchial epithelium (Confer and Ayalew, 2013; Kisiela and Czuprynski, 2009; Lo and Sorensen, 2007; Mahasreshti et al., 1997).

M. haemolytica  does not produce siderophores and utilizes a transferrin binding system of iron uptake (Rice et al., 2007). Two transferrin-binding outer membrane proteins are produced that specifically bind bovine transferrin, which allows the bacterium to extract and utilize iron from transferrin. The gene tbpA codes for the TbpA protein of 100 kDa and the tbpB gene codes for the TbpB protein of 71 kDa. In vivo, these proteins are immunogenic and are protective for experimentally infected specific pathogen free (SPF) sheep (Potter et al., 1999).

Leukotoxin 

M. haemolytica leukotoxin is one of a family of functionally and genetically similar bacterial toxins, all of which contain repeating domains in the toxin molecule and are designated RTX toxins (repeat domains in the structural toxin) (Lo et al., 1987; Rice et al., 2007). Bacteria that produce RTX toxins include Actinobacillus actinomycetemcomitans [Aggregatibacter actinomycetemcomitans], A. pleuropneumoniae, A. suis, A. equuli subspecies haemolyticus, Bordetella pertussis, hemolytic Escherichia coli strains, and others (Frey and Kuhnert, 2002). M. haemolytica leukotoxin has 6 repeat domains, which are rich in glycine and contain 9 amino acids. All M. haemolytica serotypes produce this soluble, heat-labile leukotoxin with exclusive specificity for leukocytes of ruminants by binding to CD11a/CD18 (Davies and Baillie, 2003; Larsen et al., 2009). Non-toxic isolates are rarely found. At low concentrations, this toxin impairs phagocytosis and lymphocyte proliferation; higher concentrations result in cell death due to lysis (Rice et al., 2007). Lysis results from formation of 0.9- to 1.2-mm transmembrane pores in the target cell, which allows the movement of potassium, sodium and calcium along transmembrane gradients (Clinkenbeard et al., 1989). The enzymes released following cytolysis and the leukotoxin itself are chemotactic for other leukocytes and augment lung damage due to increased cell recruitment in this area. In addition, leukotoxin is lytic for ruminant platelets, lysis of which may induce pulmonary vascular thrombosis and fibrin exudation, which is typical of shipping pneumonia (Clinkenbeard and Upton, 1991). The genes involved in leukotoxin production and secretion are designated lktA, lktB, lktC and lktD and are necessary for the production of active toxin. Gene products of lktB and lktD facilitate secretion of the toxin, and the product of lktC is involved with toxin activation (Highlander, 2001).

Extracellular enzymes

(Information from Rice et al., 2007)

At least 4 enzymes secreted by bacteria can facilitate colonization of pulmonary alveoli or reduce the effect of defensive mechanisms:

Sialoglycoprotease is a neutral protease, coded by the gcp gene and present in all the M. haemolytica strains; it specifically hydrolyses O-sialoglycoproteins associated with host cell membranes.

Neuraminidase, which hydrolyzes sialic acid from cell surface molecules, is produced by all the strains during the stationary phase of bacterial growth, and is also produced in vivo.

Superoxide dismutase, which catalyzes the conversion of superoxide radicals to hydrogen peroxide and oxygen, is produced only by serotypes 1 and 2

IgG₁ protease, which cleaves bovine IgG₁, thus reducing the efficacy of the immune response, was described in serotype 1. The enzyme is similar to or may in fact be the sialoglycoprotease, because sequence studies have failed to identify a specific gene for the enzyme.

In the USA, pneumonia caused by M. haemolytica is the main cause of economic losses in the breeding of calves. The same condition is also observed in Europe (Fels-Klerx et al., 2002). In addition, M. haemolytica is the second commonest agent causing mastitis in goats in Europe and is one of the main aetiological agents of this disease in USA. Gelasakis et al. (2015) report that sheep mastitis due to M. haemolytica and Staphylococcus aureus causes substantial economic and disease losses to the sheep industry

Financial losses that result from calf pneumonia occur due to death, treatment cost and decreased lifetime productivity (and possibly a lower grade of meat in cattle treated multiple times for the disease). Michigan dairy producers estimated that respiratory disease in calves cost them US $14.71 per calf/year (Kaneene and Hurd, 1990) while producers in California estimated that calf respiratory disease cost them US $9 per calf/year (Sischo et al., 1990). The economic loss is over US $1 billion in North American beef and dairy cattle (Miles, 2009) (Griffin, 1997).

A study in two dairies in Mexico (Pijoan and Chavez, 2003) evaluated direct and indirect costs of losses due to pneumonia. Direct costs included fatalities, discards and treatment. Indirect costs included vaccination and preventive treatment. The range varied from US $52.78 per calf to US $24.72 per calf.

In free ranging North American Bighorn sheep (Ovis canadensis), M. haemolytica (and also Bibersteinia trehalosi) cause severe pneumonia in all age groups (Besser et al., 2013). These bacteria have been largely responsible for the large decline in the Bighorn sheep population.

Because respiratory disease is the major cause of economic losses in the beef cattle industry and M. haemolytica is such a major cause of bovine pneumonia, antibiotics are used in large amounts to control the pneumonia (Rice et al., 2007).  This practice has led to concern that high antibiotic use in meat and dairy cattle can lead to antimicrobial resistance in human pathogens (Oliver et al., 2011).

Although M. haemolytica is not normally an important zoonotic agent, it can cause serious disease in human infants and immunocompromised adults; it has been demonstrated in septicaemia of infants and in adults with heart disease (Punpanich and Srijuntongsiri, 2012; Takeda et al., 2003).

• Invasive in its native range
• Proved invasive outside its native range
• Abundant in its native range
• Fast growing
• Has high reproductive potential
• Reproduces asexually
• Has high genetic variability

• Host damage
• Negatively impacts agriculture
• Negatively impacts animal health
• Negatively impacts livelihoods
• Threat to/ loss of endangered species

• Pathogenic

• Highly likely to be transported internationally accidentally
• Difficult/costly to control