Bartonella

Bartonella henselae is a fastidious and slow-growing Gram-negative bacterium causing cat scratch disease (CSD) in immunocompetent patients or bacillary angiomatosis in immunosuppressed patients. This zoonotic pathogen is distributed worldwide.

The predominant symptom of cat scratch disease is regional lymphadenopathy, which develops 2–3 weeks after exposure (with a range of 7–50 days). Characteristically, the affected lymph node is tender and clearly swollen. The most frequently affected lymph nodes are the axillary and epitrochlear nodes (46%), nodes in the head and neck (26%), and those in the groin (17.5%). In 15% of cases, the lymph node also suppurates.

Thirty per cent of patients suffer from mild fever below 39°C. B. henselae is the third most infectious cause of fever of unknown origin. Further accompanying symptoms include myalgia, malaise, and fatigue. Perinodal forms of bartonellosis may lead to endocarditis. In immunocompromised patients, infections with B. henselae can lead to bacillary angiomatosis.

Bordetella Pertussis

Bordetella species are non-spore-forming, encapsulated, bipolar, coccoid (pale-staining) Gram-negative bacilli (approximately 0.3–0.5 μm thick and 1 μm long). The genus consists of the human parasites B. pertussis and B. parapertussis, as well as B. bronchiseptica, which causes enzootic infections in various wild and domestic animal species.

Bordetella pertussis produces a single disease syndrome in humans known as pertussis or whooping cough. It is a highly contagious childhood disease (approximately 80% of cases occur before the age of five years), transmitted via respiratory contact, and is associated with a high mortality rate (around 1–2% in the first year of life, later reducing to about 0.1%). In the absence of immunisation, essentially no one escapes pertussis.

Brucella

Brucella is a small Gram-negative bacterium (0.4–0.8 μm in diameter and 0.4–3.0 μm in length) that is non-flagellated and non-spore-forming. Since the discovery of Brucella melitensis by Bruce in 1887, an increasingly complex pattern of strains has emerged, each with distinctive epidemiological features.

Virulent Brucella organisms can infect both non-phagocytic and phagocytic cells; however, the mechanisms of pathogenesis of brucellosis in its natural host species and in humans are still not completely understood. Worldwide, brucellosis remains a major source of disease in humans and domesticated animals. Although the reported incidence and prevalence of the disease vary widely from country to country (ranging from <0.01 to >200 per 100,000 population), bovine brucellosis, caused mainly by B. abortus, is still the most widespread form.

High-risk groups include abattoir workers, meat inspectors, animal handlers, veterinarians, and laboratory personnel. Brucellosis is a nationally notifiable disease and must be reported to the local health authority.

Chlamydia

Chlamydiae are non-motile, Gram-negative, and obligate intracellular bacteria that form characteristic inclusions within the cytoplasm of parasitised cells, making them easily visible under a light microscope. Three different Chlamydia species are known to be pathogenic to humans: Chlamydia trachomatis, Chlamydia pneumoniae, and Chlamydia psittaci, while C. pecorum is only pathogenic to animals. Chlamydia trachomatis is the most prevalent cause of sexually transmitted infections worldwide, with an estimated 400–500 million cases, and the number of infections is constantly increasing. In Europe, infection rates among sexually active young people commonly range between 5% and 10%. In women, Chlamydia trachomatis can lead to pelvic inflammatory disease (PID), tubal infertility, and ectopic pregnancy. Infection during pregnancy is associated with premature rupture of membranes, low birth weight, and miscarriage.

Chlamydia trachomatis can also be transmitted from mother to baby during labour, leading to eye and respiratory infections. In men, Chlamydia trachomatis can cause acute genital inflammation (epididymitis, epididymo-orchitis) and, in some cases, sexually acquired reactive arthritis (SARA). In both men and women, Chlamydia trachomatis may lead to proctitis. Individuals infected with Chlamydia trachomatis are at an increased risk of acquiring or transmitting HIV. Extra-articular infection with Chlamydia trachomatis can cause inflammatory reactive arthritis (Chlamydia-induced arthritis, CIA).

A major concern with Chlamydia infections is their frequently asymptomatic and insidious progression, which may contribute to chronic disease. In many cases, primary infections go unnoticed, and only the long-term complications caused by ascending, persistent bacteria are diagnosed. After primary infection, IgM, IgA, and IgG antibodies appear in serum successively. IgG antibodies are generally considered markers of past exposure to the pathogen, regardless of disease stage. IgM antibodies indicate acute infection, while IgA antibodies suggest ongoing progression. C. pneumoniae causes respiratory diseases such as pneumonia and bronchitis and is also suspected to contribute to conditions such as endocarditis and coronary heart disease.

Haemophilus Influenzae (HIB)

Haemophilus influenzae type B (HiB) is a very common cause of invasive, critical infectious diseases in children up to the age of six. Following infection, the symptoms of the disease include pericarditis, osteomyelitis, meningitis, encephalitis, pneumonia, sinusitis, and otitis. In many cases, the disease is lethal or leads to neurological damage, which cannot always be prevented by rapid antibiotic therapy. The underlying reason for the disease is often a latent immunodeficiency with a specifically reduced humoral immune response to the polyribosylribitolphosphate (PRP) in the polysaccharide encapsulation of the bacterium. In children, another reason is the immaturity of the immune system.

Today, the term “immunocompromised patients” is often used to refer to individuals with both acquired and innate specific and nonspecific immunodeficiencies. As a result, in children aged 3 months or older, vaccination with various types of PRP-containing vaccines is recommended. This can lead to a significant reduction in the number of infections caused by Haemophilus influenzae type B. The titre of antibodies produced by vaccination can be used to confirm whether the vaccination has been successful. HiB IgG ELISA is used to measure the level of PRP-specific IgG antibodies following a 4-6 week period after complete immunisation, to monitor the humoral immune status of children or other individuals at risk.

Helicobacter Pylori

Helicobacter pylori is a spiral Gram-negative bacterium (2 μm to 6.5 μm in size, flagellated) which colonises the human gastric mucosa. The organism is found in the mucus layer and adheres to the surface mucus epithelium of the stomach but generally does not penetrate the gastric mucosa directly. However, there is a secondary inflammatory response in the mucosa leading to chronic active gastritis.

Helicobacter pylori is the primary causative agent in most cases of peptic ulcer disease. In 1994, the WHO classified Helicobacter pylori as a category 1 carcinogen. The infection rate in Europe is about 30%-40%, and worldwide about 50%. There is an inverse relationship between the presence of Helicobacter pylori infection and socioeconomic status. In developing countries, people acquire the infection at an early age such that by young adulthood, as many as 90% of the population might have Helicobacter pylori gastritis. In developed western countries, the prevalence of Helicobacter pylori gastritis is much lower. Under these conditions, the rate of acquisition is much slower (roughly 1% per annum), and the older one is, the more likely one is to be infected with the organism.

Meningitis

Neisseria meningitidis, also known simply as meningococcus, is a heterotrophic gram-negative diplococcal bacterium, best known for its role in meningitis and other forms of meningococcal disease. It exclusively infects humans and is the only form of bacterial meningitis known to cause epidemics. The bacterium, which can spread from person to person, typically causes initial colonisation in the upper airway, often without symptoms. From there, it can penetrate into the bloodstream, leading to the central nervous system and causing meningitis, or it may develop into a full-blown bloodstream infection (meningococcemia).

Twelve subtypes or serogroups of N. meningitidis have been identified, five of which (A, B, C, Y, and W135) are recognised as causing epidemics. The pathogenicity, immunogenicity, and epidemic potential vary according to the serogroup. Therefore, identifying the responsible serogroup in a sporadic case is crucial for epidemic containment. The meningococcal vaccines currently approved for use in humans are made from the variant, purified capsular polysaccharides, which are characteristic of the bacterial membrane. Vaccines are available for serogroups A, C, Y, and W135, but not for serogroup B. Antibodies against the meningococcal capsular polysaccharides (MCP) are protective in adults and children aged over 2 years, and antibodies have been detected up to four years after vaccination.

Mycoplasma Pneumonia

The mycoplasmas belong to the class Mollicutes, which comprises three distinct families and four genera, one of which is Mycoplasma, containing over 60 species. Mycoplasmas are the smallest free-living organisms known (300 to 500 nm in diameter) and, unlike regular bacteria, they lack a cell wall. Mycoplasmas are extracellular parasites, particularly on mucous membranes, and can cause infections in humans, animals, plants, and cell cultures.

Mycoplasma pneumoniae is primarily a respiratory pathogen (obligate) in humans, affecting the nasopharynx, throat, trachea, bronchi, bronchioles, and alveoli. Other mycoplasmas, including M. buccale, M. faucium, M. orale, and M. salivarium, are commensals in the oral cavity. Mycoplasma hominis and Ureaplasma urealyticum primarily inhabit the genital tract and may act as opportunistic invaders. M. pneumoniae is by far the most important pathogen in this group.

Infection with M. pneumoniae occurs worldwide, with epidemiological studies primarily conducted in the USA, Europe, and Japan. Infections are endemic in larger urban areas, and epidemic increases occur at varying intervals. M. pneumoniae has been estimated to cause 15-20% of all pneumonias, with the highest rates observed in children and young adults. Seventy-four per cent of M. pneumoniae infections are asymptomatic, and reinfection may occur. Naturally acquired immunity to M. pneumoniae infection appears to be of limited duration, typically lasting 2-3 years.

Treponema Pallidum (Syphilis)

Spirochetes are motile bacteria characterised by a periplasmic axial filament. All pathogenic species belong to the family Treponemataceae, which includes three genera: Treponema, Borrelia, and Leptospira. Treponema species are motile bacteria, measuring 5-15μ in length and 0.2μ in width, containing about 10 flexible, undulating, spiral-shaped rods. Treponema pallidum, the causative agent of syphilis, is transmitted through direct contact, usually via sexual intercourse. Syphilis, along with gonorrhoea, chancroid, and lymphogranuloma venereum, is designated as a venereal disease (VD), and is an acute and chronic infectious condition.

After an incubation period of 12-30 days, the first symptoms to appear are chancres, followed by syphilitic ulcers that typically disappear spontaneously within a few weeks. During this first stage (primary syphilis), Treponema pallidum propagates to nearby lymph nodes and is then distributed throughout the bloodstream. Three further stages of the disease follow, which are classified as secondary, tertiary, and quaternary syphilis.

Treatment with antibiotics during the earliest stage of the disease, as well as prophylactic measures, are essential to prevent epidemics. For this purpose, antenatal and blood donor screenings are mandatory in most countries worldwide.