Bacteria and viruses you may not know

PRIONS

Prions are a composition of abnormal prion proteins found in neurons, that are resistant to protease. They are about 30kD in size, and are also normally found in neurons as prion proteins. Acquired or inherited mutation of prion proteins make them resistant to protease, resulting in their accumulation in the brain.

They cause Creutz-Feldt Jakob disease (CJD) and variant CJK which are also iatrogenically transmissible or transmitted from infected cow respectively.

VIRUSES

Viruses are obligate intracellular parasites, that depend on host cell to replicate. Both DNA and RNA viruses have a protein coat (capsid) surrounding them. Their sizes range from 20nm to 300nm. We shall discuss briefly of various viruses.

Measles virus

The measles virus is a single-stranded RNA virus that causes acute infection in cells. They have an outer lipid membrane envelope derived from human cell membrane. Transmission occurs through respiratory droplets, with infection initially occurring in upper respiratory epithelial cells, and then to local lymphoid tissues.

The virus has surface hemagglutinin proteins that enable entry into host cells. The worldwide mortality and morbidity rates are about 35%, 20% respectively. Children in developing countries bear the most burden of the disease. For, this reason, a vaccine is available to prevent both the morbidity and mortality.

Clinical manifestations: diarrhea; croup; pneumonia; keratitis and scarring; blindness; encephalitis; koplik spots around parotid duct; hemorrhagic rashes on face, trunk and proximal extremities. Viral culture and serology testing offer accurate diagnosis.

Mumps virus

This is a single-stranded RNA virus that causes acute infection in cells. They have an outer lipid membrane envelope derived from human cell membrane. Transmission is through respiratory droplets, with infection initially occurring in upper respiratory epithelial cells, and then to local lymphoid tissues.

The mumps virus possesses hemagglutinin and neuraminidase surface glycoproteins that enable entry into host cell. They infect salivary gland and other glands when they spread in blood. The mumps vaccine is available to reduce incidence of infection.

Clinical manifestations: parotitis; aseptic meningitis; pancreatitis; orchitis; encephalitis.

Viral culture and serology testing are useful in diagnosis.

Poliovirus

This is an unencapsulated RNA virus that causes acute infection in host tissues, especially children. They are transmitted by fecal-oral route, and initially infect epithelia of oropharynx. They are secreted in saliva and then swallowed to infect intestinal mucosa. Poliovirus also infect CNS and spinal cord motor neurons.

Clinical manifestations: fever; diarrhea; rashes; bulbar poliomyelitis; spinal poliomyelitis. Viral culture of throat secretions, stool and serology offer accurate diagnosis. Two vaccines are available: attenuated live vaccine and killed vaccines.

West Nile virus

This is an encapsulated RNA virus, that causes acute infection and is spread by mosquito bites. Humans are incidental hosts, while wild birds are reservoir hosts.

Clinical manifestations: fever; meningitis; headache; myalgia; encephalitis; meningoencephalitis; hepatitis; pancreatitis; myocarditis.

Diagnosis is usually by serology test.

Marburg, Lassa, & Ebola virus

These are RNA viruses with surrounding outer lipid membrane layer that cause acute infections.  They cause viral hemorrhagic fever (VHF). Transmission occurs through contact with someone with infection or bite from an insect vector.

Clinical manifestations: fever; myalgia; headache; rash; thrombocytopenia; neutropenia; shock.

For diagnosis, use serology testing.

Herpes simplex virus 1 & 2 (HSV-1, HSV-2)

These are DNA viruses, with an outer lipid membrane covering layer. They both cause acute and latent infections. After infecting epithelial cells, they spread to infect nucleus of surrounding sensory neuron. Spread by skin contact with someone having infection. 

Clinical manifestations: cold sores around lip, nose; gingivostomatitis; herpetic whitlow; keratitis; genital herpes (common in HSV-2); herpes hepatitis and bronchopneumonia; eczema herpeticum (pustular/hemorrhagic blisters over skin) Performing viral culture is useful in diagnosis.

Varicella-Zoster virus

This is a DNA virus, with an outer lipid membrane covering layer. It causes both acute and latent infection. Reactivation of latent infection causes shingles with a more painful lesion, especially around the face.

Transmission is airborne, and may also be through skin contact. They first infect epithelial cells of respiratory tract, and then sensory dorsal root ganglion of neurons.

Clinical manifestations: fever; chickenpox rash (from trunk to face and then extremities); interstitial pneumonia; encephalitis; myelitis; shingles (itching, sharp, burning pain).

Performing viral culture is useful in diagnosis.

Cytomegalovirus

This is a DNA virus, with an outer lipid membrane covering layer. Transmission occurs by saliva, sexually, iatrogenically, transplacentally, transcervically. They cause acute and latent infections in epithelia and monocytes respectively, where they form intranuclear basophilic inclusions.

Clinical manifestations: neonates with infection show encephalitis; thrombocytopenia; bleeding; anemia; hepatosplenomegaly; jaundice; interstitial pneumonitis; skin rash; hepatitis.

Other manifestations include: atypical lymphocytosis; lymphadenopathy; fever; hepatitis; hepatomegaly.

Virus culture of urine or oral secretions for neonatal infections, with serology tests for young adults offer diagnostic results.

Hepatitis B virus

This is a DNA virus with an outer lipid membrane covering layer. Transmission occurs through intravenous drug use; iatrogenically; transplacentally. It infects hepatocytes and initiates immune response, where it causes both acute and chronic infection.

Clinical implications: hepatitis; liver cirrhosis; hepatocellular carcinoma.

Epstein-Bar Virus

This is an enveloped DNA virus, that causes infectious mononucleosis, a benign self-limiting lymphoproliferative disorder. Transmission occurs through contact by kissing. It crosses mucosal epithelium of mouth to pharyngeal and palatine tonsils, where it acutely and latently infects B lymphocytes.

Clinical manifestations; lymphadenopathy (axillary, groin, posterior cervical region); fever; sore throat; malaise; fatigue; atypical lymphocytosis (cytotoxic T cells); splenomegaly; hepatomegaly.

Complications: liver failure; jaundice; splenic rupture from minor trauma; agammaglobulinemia; nasopharyngeal carcinoma; B cell lymphoma; Burkitt lymphoma.  Anti-Epstein Bar virus nuclear antigens test, and monospot test (heterophile antibody reaction) are useful in diagnosis.

GRAM POSITIVE BACTERIA

These are prokaryotes with a cell membrane, but lack membrane bound nuclei and organelles. Two major types are: gram positive and gram-negative bacteria due to crystal-violet stain.

Their shapes may be rod or spherical like, which one may call bacilli or cocci respectively. Bacteria have sizes that span from 0.2µm to 15µm. They may be obligate intracellular, facultative intracellular, or extracellular.

Gram positive bacteria have outer thick peptidoglycan cell wall surrounding their phospholipid cell membrane, which retains crystal violet stain.

Gram negative bacteria have thin peptidoglycan cell wall between two phospholipid cell membrane, and doesn’t retain crystal violet stain.

Staphylococcus aureus, S. epidermidis, S. saprophyticus

These are pyogenic, spherical gram-positive bacteria, that individually form grape-like clusters. S. epidermidis have membrane bound capsules, and causes opportunistic infections in patients with prosthetic cardiac valves, catheters, & drug abusers. S. saprophyticus causes urinary tract infections in young women.

Staphylococcus aureus produces superantigens, exfoliative A & B toxins. These superantigens cause food poisoning and toxic shock syndrome (TSS). The exfoliative toxins are responsible for bullous impetigo and scalded skin syndrome (SSS) which are superficial epidermal lesions.

Clinical implications of S. aureus: boil (furuncle), carbuncle, hidradenitis; paronychia; pneumonia after influenza infection; abscess; sepsis; endocarditis; osteomyelitis; food poisoning; vomiting; TSS; SSS.   

Mode of transmission can be: contact with contaminated objects, food, & skin. To diagnose, bacterial culture is essential.  

Streptococcus pyogenes, S. agalactiae

These are spherical gram-positive pyogenic bacteria that exist either in pairs or chains. They are β-hemolytic streptococci and their transmission occur through contact with contaminated surfaces or airborne (as in S. pharyngitis).  Rapid strep test and bacterial culture can diagnose the bacterial infection.

Group A S. pyogenes causes: pharyngitis; scarlet fever; erysipelas; impetigo; TSS; acute rheumatic fever; glomerulonephritis.

Group B S. agalactiae is a normal flora of female genital tract and causes: neonatal sepsis and meningitis; chorioamnionitis.

Streptococcus pneumoniae, S. mutans, viridans streptococci

These are spherical gram-positive pyogenic bacteria that exist either in pairs or chains.

α-hemolytic S. pneumoniae causes community acquired lobar pneumonia. Transmission occurs through contact with infected mucus or inhalation.

S. mutans is a normal oral flora and the major cause of dental caries, through demineralization of tooth enamel.

Viridans streptococci consist of a large group of alpha & non-hemolytic gram-positive bacteria. They are normal oral flora and usually cause endocarditis.

Enterococci

These are spherical gram-positive pyogenic bacteria that exist either in pairs or chains. They cause endocarditis and urinary tract infections and are usually antimicrobial multidrug resistant.

Corynebacterium diphtheria, Listeria monocytogenes

These are gram-positive rod bacteria. C. diphtheria causes diphtheria, while L. monocytogenes causes listeriosis.

C. diphtheria is mainly an extracellular bacterium, and spreads by inhalation and skin contact. It causes: epithelial necrosis of respiratory tract; parenchymal necrosis of liver, kidney, adrenals, myocardial necrosis; polyneuritis.

L. monocytogenes is a facultative intracellular microbe. Spreads by food contamination (chicken, dairy products, hot dogs); transplacentally. It causes food poisoning; abortion; sepsis; still birth; exudative meningitis.

Bacillus Anthracis, Nocardia

These are gram positive rod bacteria. Bacillus anthracis causes anthrax, while Nocardia causes nocardiosis.

Bacillus anthracis is a spore forming bacteria. Transmission is by contact with farm or wild animals; inhalation of dust containing bacterial spores; undercooked meat.

It causes: pruritic papules, hemorrhagic mediastinitis; nausea; vomiting; bloody diarrhea; fever; cough; chest, abdominal pain; sweating; hypoxia; meningitis respiratory distress; shock.

Nocardia species include N. asteroides & N. brasiliensis. They are found in soil and transmission occurs by inhalation and direct contact. N. asteroides causes respiratory and CNS infections. N. brasiliensis causes skin infection.

Clostridium species

These are gram positive anaerobic bacilli bacteria, that release spores found in the soil. They include: C. perfringens; C. difficile; C. botulinum; C. tetani.

C. perfringens causes myonecrosis of surgical or traumatic wounds; uterine myonecrosis; mild food poisoning.

Responsible for pseudomembranous colitis is the C. difficile in people taking antibiotics.

C. botulinum from improper food can sterilization causes respiratory and skeletal muscle paralysis due to neurotoxin release.

C. tetani infects puncture wounds and releases tetanospasmin, responsible for convulsive contraction of skeletal muscle (lockjaw).