Fundamentals of atypical bacteria

Rickettsiaceae, Coxiellaceae and Bartonellaceae

The families Rickettsiaceae, Coxiellaceae and Bartonellaceae include a number of clinically important genera, Rickettsia, Coxiella and Bartonella.

Although these are prokaryotic cells, they differ from most other bacteria both in their structure and in the fact that
the majority of species lead an obligate intracellular existence.

This means that, with a few exceptions,
they cannot be grown on cell-free media, although unlike many viruses they do possess some independent enzymes.

They have a pleomorphic appearance, ranging from coccoid through to rod-shaped cells; multiplication is by binary fission.

Their cell wall composition bears similarities to that of Gram-negative bacteria and in general they stain this way.

The genus Rickettsia has a number of species that give rise to human diseases, in particular epidemic typhus (Rickettsia prowazekii), murine typhus (Rickettsia typhi) and spotted fevers (various species).

These are characterized by transmission via insect vectors, particularly mites, ticks, fleas and lice.

The mode of transmission by these vectors varies depending on the insect concerned.

In the case of lice and fleas, the microorganisms multiply within the insect and get into the faeces.

These insects then colonize humans and transmit the microorganism when the faeces or the insect itself is crushed onto
the skin.

No bite is necessary, and the faeces may
also be inhaled.

Mites and ticks pick up the microorganism when they take a blood meal from an infected animal.

They then pass on the infection to
humans when they accidentally bite us.

Coxiella burnetii is the only species in the genus Coxiella and it gives rise to a disease called Q fever.

Although the source of the disease is infected animals, usually no insect vector is involved, and the most common route of transmission is by inhalation of
infected dust.

Bartonella quintana is the causative
agent of trench fever, which, as the name suggests, occurs typically under conditions of war and deprivation.

Each of the infections described here can be treated with the antibiotic doxycycline, although the duration of therapy may vary depending upon the
nature of the disease and its severity.

Chlamydiae

These are obligate intracellular parasitic bacteria that possess some independent enzymes but lack the ability to generate ATP.

Two cellular forms are identified: a small (0.3 µm) highly infectious elementary body, which, after infection, enlarges to give rise to the replicative form called the initial or reticulate body (0.8 µm to 1.2 µm).

This divides by binary fission within membrane-bound vesicles in the cytoplasm of infected cells.

Insect vectors are not required for the transmission of infection. Chlamydiae lack peptidoglycan in their cell walls and have weak Gram-negative characteristics.

Chlamydia trachomatis is a clinically important member of the group, being responsible for the disease trachoma, characterized by inflammation of the
eyelids, which can lead to scarring of the cornea.

This is the most common cause of infectious blindness worldwide. It is estimated that 400 million people
are infected, with at least 6 million totally blind.

The same species is also recognized as one of the major causes of sexually transmitted disease.

Chlamydophilia psittaci and Chlamydophilia pneumoniae are responsible for respiratory tract infections.

Chlamydial infections are responsive to treatment with tetracyclines, administered either topically or systemically as appropriate.

Mycoplasmas

The mycoplasmas are a group of very small (0.3 µm to 0.8 µm) prokaryotic microorganisms that are capable of growing on cell-free media but which lack cell walls.

The cells are surrounded by a doublelayered plasma membrane that contains substantial amounts of phospholipids and sterols.

This structure has no rigidity owing to the absence of peptidoglycan, and so the cells are susceptible to osmotic lysis.

The lack of peptidoglycan is also the reason for these bacteria being resistant to the effects of cell-wallacting antibiotics such as the penicillins, and also the
enzyme lysozyme.

Members of this group are called
pleomorphic, which means they can vary in shape, and these cells range from coccoid to filamentous.

Most are facultative anaerobes capable of growth at 35°C, and on solid media produce colonies with a characteristic ‘fried egg’ appearance.

They contain a number of genera, of which the most important from a clinical point of view are Mycoplasma and Ureaplasma.

Mycoplasma pneumoniae is a major cause of respiratory tract infections in children and young adults, whereas Ureaplasma urealyticum has been
implicated in nonspecific genital m infections.

Despite being resistant to the β-lactam antibiotics, these infections can be effectively treated using either
tetracyclines or erythromycin.

Actinomycetes

Many of the macroscopic features of the actinomycetes are those that are more commonly found among the filamentous fungi but they are indeed prokaryotic cells.

They are a diverse group of Gram-positive bacteria morphologically distinguishable from other bacteria because they have a tendency to produce
branching filaments and reproductive spores.

Actinomyces israelii is the most common cause of actinomycosis, which can manifest itself as abscesses in the oral cavity or gastrointestinal tract.

It may also cause endocarditis. The genus Nocardia contains a number of species that have been shown to be pathogenic to humans, but they are of low virulence
and infect mainly immunocompromised patients.

Reproduction in this genus is by fragmentation of the hyphal strands into individual cells, each of which
can form a new mycelium.

The genus Streptomyces contains no human pathogens, and most species are
saprophytic bacteria found in the soil.

They are aerobic microorganisms producing a nonfragmenting, branching mycelium that may bear spores.

The reason for their pharmaceutical importance is their ability to produce a wide range of therapeutically useful
antibiotics, including streptomycin, chloramphenicol, oxytetracycline, erythromycin and neomycin.

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