Infection and immunity

Structure ( Fig. 15.1 )

Viruses consist of a core of nucleic acid surrounded by a protective protein coating known as the capsid. The capsid mediates the attachment of the virus to specific host cell receptors and defines the species and organ specificity of the virus. The capsid may induce host immune responses. In some viruses, the capsid is covered by a lipoprotein envelope, which confers instability to the virus. Enveloped viruses, such as RSV, dry out rapidly in the environment and are easily inactivated by detergent and alcohol. Viruses without an envelope, such as rotavirus or Norwalk virus, are less easily eradicated in the environment. The classification of viruses is based on the viral nucleic acid (DNA or RNA), capsid (size, symmetry) and presence of envelope.

Pathogenesis

Viral infection of a host cell may result in a number of consequences:

• Cell death due to inhibition of host cell protein synthesis while allowing ongoing viral protein synthesis. This may be visualized with light microscopy as the cytopathic effect (CPE) in cell cultures, aiding diagnosis.

• Fusion of host cells to form multinucleated giant cells, which can be visualized in cell cultures. For example, fusion protein of RSV results in syncytia through this mechanism.

• Malignant transformation by causing unrestrained growth, prolonged survival and morphological changes of the cell, for example, human papillomavirus.

• Viral replication without killing of the host cell can result in chronic carriage, for example hepatitis B. Disease may result from the host inflammatory response against viral antigens expressed on the host cell surface rather than from direct damage by the virus itself.

• The virus may remain within the cell but not replicate and latent infection ensues – a feature of all herpesviruses. Varicella zoster virus (VZV) enters a latent phase following primary infection and later causes shingles when reactivated.

Classification

Bacterial classification is based on four features: Gram reaction, bacterial shape (see Fig. 15.2 ), growth requirement and the presence of spores ( Table 15.1 ). Most bacteria grow in the presence of oxygen (aerobes), some require it (obligate aerobes) whilst others can still generate energy in the absence of sufficient oxygen (facultative aerobes). A number of bacteria will only grow in an atmosphere containing less than 20% oxygen (anaerobic). Some bacteria produce spores in adverse conditions, allowing the organism to survive when exposed to chemicals and heat (i.e. Clostridium species).

A number of important pathogenic bacteria do not fit neatly into this classification as they do not take up the Gram stain: Mycoplasma (no cell wall), Chlamydia and Rickettsia (intracellular bacteria) and Mycobacteria (acid fast staining only).

Structure

Bacterial cells consist of cytoplasm surrounded by a cell wall. The DNA is free within the cytoplasm as a single chromosome of circular DNA and within plasmids, along with ribosomes and all elements required for growth and pathogenesis.

The cell wall is essential for survival and is a key target for antibiotics; differences in the components of the cell wall between Gram-negative and Gram-positive bacteria ( Fig. 15.3 ) are therefore clinically important (see antimicrobial section). Gram-positive bacteria have a thick peptidoglycan layer with no outer membrane, whereas Gram-negative bacteria have a thin peptidoglycan layer surrounded by an outer lipid membrane. The periplasmic space of Gram-negative bacteria may contain β-lactamase, which degrades anti­biotics such as penicillin.

The cell surface may contain different components. Gram-negative bacteria have endotoxin (lipopolysaccharide, LPS) in their outer membrane, which can induce septic shock. Teichoic acid is only found in Gram-positive bacteria and this can also induce septic shock. Numerous pili, hair-like structures, facilitate adhesion and acquisition of external DNA. Flagella are important in locomotion and may help with bacterial identification. Other proteins act as sensors, receptors and adhesins.

Bacteria such as Streptococcus pneumoniae, Neisseria meningitidis, Klebsiella pneumoniae and Escherichia coli are surrounded by a polysaccharide capsule, which enables them to evade phagocytosis. The spleen forms an important role in clearing these bacteria, therefore individuals who are hyposplenic, such as children with sickle cell, are more susceptible to these organisms.

Some bacteria produce slime in addition to the capsule and this helps with the formation of biofilms. This tough protective matrix is very difficult for antibiotics to penetrate and may form on foreign material.

Pathogenesis

Bacteria may be transmitted via the respiratory, gastrointestinal, urogenital or cutaneous route. Once transmitted, bacteria adhere to mucosal sites, facilitated by pili and surface proteins. Once a stable population of bacteria has been established, the host is colonized. In some instances, invasion occurs and the bacteria penetrate host cells and tissues. Not all strains of bacteria are equally pathogenic, for example, there are six serotypes of Haemophilus influenzae , but type b (Hib) causes the most serious disease. Moreover, different strains with differing virulence determinants cause distinct patterns of infection; for example, E. coli may cause disease in the gastrointestinal tract, meningitis, sepsis or a UTI.

Protozoa

Protozoa are unicellular organisms that can exist in a vast range of environments. The cytoplasm is surrounded by a plasma membrane, which may have external structures such as a cell wall to enable the organism to survive outside the host ( Giardia intestinalis ) or flagella ( Leishmania ) to propel the protozoa.

They can be divided into three main groups that cause disease in humans:

• 1.

  Spore-forming (sporozoa): Plasmodium , Toxoplasma gondii

• 2.

  Flagellates: Giardia intestinalis and Trichomonas

• 3.

  Amoeboid: Entamoeba histolytica (causes amoebic dysentery or liver abscess)

They reproduce sexually and/or by binary fission. Their life cycle may involve vectors; for example, Plasmodium parasites are transmitted by mosquitoes (vector) and cause malaria in humans. Other protozoa are waterborne and only involve humans ( Entamoeba histolytica ).

Fungi

Fungi are widespread in the environment and can survive in hostile environments. They are saprophytes, living off dead matter in soil and water. Their cell wall contains chitin polysaccharide and not peptidoglycan. As a result, fungi are not sensitive to antibiotics that inhibit peptidoglycan synthesis. The cell wall also contains the polysaccharide β-glucan and ergosterol, which are targeted by various antifungal drugs (see antimicrobial section).

Fungi can be classified into yeasts, moulds and dimorphic fungi.

Yeasts are simple unicellular organisms that reproduce by asexual budding, Candida albicans is responsible for most disease caused by yeasts in humans.

Moulds grow as long filaments. Their branching filamentous hyphae assist with reproduction and acquisition of nutrients. They produce germinative spores, which enable them to colonize new environments. Airborne spores of Aspergillus fumigatus may be inhaled and cause infection in immunocompromised hosts.

Dimorphic fungi take the form of moulds at room temperature, but transform into yeasts at body temperature. Histoplasma capsulatum is a dimorphic fungus that may cause disease in individuals with HIV infection.

Fungal infections (mycoses) mainly cause superficial infections that are localized to epidermis (tinea corporis if it affects the body, tinea pedis if it affects the feet), hair (tinea capitis) and nails (tinea unguium). All forms of tinea that affect the skin may be referred to as ringworm; these are typically caused by dermatophytes. Dermatophytes belong to one of three fungal groups: Trichophyton, Microsporum and Epidermophyton . Systemic infections are most commonly due to opportunistic fungi in immunocompromised hosts.

Helminths

Helminths are complex multicellular parasitic worms that range in size from microscopic filarial parasites to tapeworms several metres in length. They reproduce sexually and have complex lifecycles. Helminths typically cause chronic rather than acute diseases.

They can be classified into nematodes (round worms) and platyhelminths (flatworms), which include cestodes (tapeworms) and trematodes (flukes).

Nematodes appear worm-like and cause infection in the intestine ( Enterobius vermicularis, causing pruritus ani in children), blood ( Filaria , such as Wuchereria bancrofti causing lymphatic filariasis) and tissues ( Onchocerca volvulus, causing river blindness).

Cestodes (e.g. Taenia solium, T. saginata ) are ribbon-like worms and can grow up to 10 m in length. The excreted eggs are ingested by an intermediate host, such as a cow or pig; humans become infected by eating meat from this animal.

Trematodes are flat, leaf-like organisms. Humans are the definitive host and freshwater snails are the intermediate host. Schistosoma species are a medically important example of trematodes.

i)

Disruption of bacterial cell wall

β-Lactam and glycopeptide agents prevent cross-linkage of peptidoglycan, a key component of the bacterial cell wall. The bacterium is then killed by osmotic lysis. β-Lactams such as penicillin are predominantly used to treat Gram-positive infections caused by Streptococci . Third-generation cephalosporins such as ceftriaxone are active against a much broader spectrum of bacteria, including Gram-positive and Gram-negative organisms; however, ceftriaxone has poor action against Pseudomonas and Enterococci . Glycopeptide activity is limited to Gram-positive bacteria, as the large molecules are not able to penetrate the outer membrane of Gram-negative bacteria.

ii)

Inhibition of protein synthesis

Protein synthesis is inhibited by macrolides, tetracylines, aminoglycosides and clindamycin at the level of the ribosome. As a group, these antibiotics are active against a wide range of bacteria. Macrolides have a similar spectrum of activity as penicillin but Mycoplasma, Mycobacteria and Chlamydia are also sensitive to macrolides. Aminoglycosides have excellent Gram-negative activity.

iii)

Inhibition of DNA replication

Fluroquinolones inhibit enzymes involved in the coiling and uncoiling of DNA, thereby inhibiting DNA replication. Fluroquinolones have good activity against Gram-negative organisms, but poor activity against Gram-positive organisms, such as Streptococci and Staphylococci .

iv)

Interruption of microbial chemical pathways

Bacteria produce folate for the synthesis of DNA as they cannot absorb folate from the host. Trimethoprim inhibits the conversion of dihydrofolate to tetrahydrofolate, thereby preventing purine and pyrimidine metabolism and DNA formation. It is active against Gram-negative and Gram-positive organsims and is most commonly used to treat urinary infections due to its excretion and high concentrations in the urine compared to blood.

Antibiotics may be considered as narrow or broad spectrum according to the range of bacteria they are active against. Broad-spectrum antibiotics should be reserved for when a wide range of bacteria could be responsible for an infection or when polymicrobial infection may be present. Antibiotics such as β-lactams and aminoglycosides are bactericidal and kill the bacteria they are effective against; these should be selected for serious infections or immunosuppressed patients.

Bacteriostatic antibiotics, such as tetracyclines or trimethoprim, inhibit bacterial growth but do not kill them, and therefore rely on the immune system to erradicate the organism.

Resistance

A bacterium is considered resistant when its growth cannot be inhibited by a concentration of drug that is achievable in the blood. Resistance may be innate; for example, Pseudomonas is innately resistant to penicillin. Alternatively, resistance may be acquired as a result of genetic change. If genetic change results in a survival advantage, then the population of resistant bacteria may outgrow the sensitive population. Antibiotics exert a considerable selection pressure on bacterial populations, favouring populations that are able to withstand them. Inappropriate and overuse of anti­biotics is the main driver for the emergence of resistant bacteria and given the shortage of new antibiotics, it is essential to maintain the efficacy of current drugs. The principles of selection of antibiotics (see Key points) and of antimicrobial stewardship ( Box 15.2 ) should be followed to ensure reduction in the inappropriate use of antibiotics.