Non-spore forming gram-positive rods:
Corynebacterium diphtheriae :
Important properties:
· Gram-positive rods that display pleomorphism, arranged as Chinese letters (V, L-shaped), swelling at the ends that give club-shaped.
· The rods have metachromatic granules (storage for high-energy phosphate bonds).
· Non-motile .
· Non-capsulated.
· Catalase positive and oxidase negative.
Habitat and transmission:
Humans are only natural host. The organisms residue in URT (nasopharynx) of carriers and in skin .Transmission of organism by respiratory droplets from person to person or it is less frequently spread by cutaneous contact with carriers and contaminated fomites.
Diphtheria toxin(DT)and pathogenesis.
The major virulence factor is exotoxin known as diphtheria toxin(DT). The production of toxin is controlled by genes carried on nucleic acid of bacterial viruses(bacteriophage)during the lysogenic phase, therefore not all strains of C.diphtheriae are toxigenic, but non-lysogenic strains are non-toxigenic.
The diphtheria toxin is A-B toxin .The toxin is synthesized as single polypeptide chain which consisting of two fragments; fragment-A(active part) and fragment-B(binding part). Both fragments are linked together by disulfide bond. Single bacterium can produce 5000 toxin molecules per hour.
Fragment-B is binding to specific surface receptor on cell membrane of host cell, and then is taking into cell by endocytosis. Because low pH inside the endosome , the enzymatic fragment-A is separated and released into cytosol, which stimulate ADP-ribosyl-transferase to cleave NAD(nicotinamide adenine dinucleotide) and transfer ADP-R(adenosine diphosphate-ribose) to cellular translation elongation factor-2 (EF-2). The ribosylation of EF-2 leads to inactivating it, which resulting in inhibition of protein synthesis , and death of cell.
Pathogenicity and clinical features:
Four biotypes are identified ; gravis, mitis , intermedius and belfanti. They cause localized and systemic diseases. Two forms of diphtheria are seen; pharyngeal and cutaneous disease.
· Acute respiratory disease(Pharyngeal diphtheria) :
The toxin destroys epithelial cells and polymorphs, and ulcer forms which is covered with necrotic exudates ,cell debris of mucosa and inflammatory products forming a false-membrane (pseudomembrane) gray , thick, adherent membrane over the tonsils and throat. These soon become dark and malodorous. Mechanical obstruction of airway by pseudomembrane may cause suffocation (asphyxia) and death of patient. Tracheostomy may be required in severe cases. pharyngitis and tonsillitis are common manifestation.
Toxemia-induced myocardial and neurologic damage occur as complications. The toxin inhibits protein synthesis in cardiac muscle can cause death, the toxin also can affect both peripheral and cranial nerves as demyelination (removal of myelin sheath) resulting paralysis of the muscle of palate and pharynx can lead to regurgitation of fluid through the nose.
Cervical lymph nodes (cervical adenitis) may be enlarging to give bull-neck appearance.
Clinical features include fever, croup, sore throat, cervical adenopathy. Difficult in speaking (nasal speech),in swallowing (dysphagia),in breathing (dyspnea).
The diphtheria may be life-threatening illness. Death can result from mechanical obstruction of airway by pseudomembrane or by toxemia-induced myocardial and neurological damage.
· Cutaneous diphtheria; toxigenic and non-toxigenic strains can colonize small break in skin, especially damage caused by insect bites or any wound in skin, and the organism produce lesion. The lesion may appear as simple pustules or chronic, non-healing necrotic ulcer. The systemic affects may occur during cutaneous lesion due to absorption of toxin into the tissues.
Other species are opportunistic pathogens and cause diphtheroid disease in immuno-compromised individuals (such as the species C.pseudodiphthericum).
Propionibacterium acnes is anaerobic species. It produces lipase enzyme which split of fatty acids of skin and cause acne.
Lab. Dx;
· Initial diagnosis is made solely on basis of clinical symptoms.
· Confirmation of diagnosis is made by isolation and identification of etiological agents as toxigenic C.diphtheriae. Swabs from nose and throat must be obtained before antibiotic administration. Gram stain showing Chinese letter, and Albert stain is done for demonstration of metachromatic granules. Culturing on Loeffler slope and blood tellurite media.
· ELISA is used to detection of antibody against antigen of this organism in serum.
· Toxin production can be determined by in vitro precipitin test (Eleck test) and in vivo animal protection test(Schick test) for detection of toxigenic strain.
· PCR for detection of tox gene.
Control:
· Treatment of choice is immunization by antitoxin, which should be given immediately and cannot wait for laboratory results. The toxin binds rapidly and irreversible to cell, once bound, cannot be neutralized by antitoxins. Therefore, the antitoxins are neutralizing the unbound toxin(these antibodies active against B-subunit by prevention of B-binding to receptor). Because the antiserum is made in horses, the patient must be tested for hypersensitivity and medications for treatment of anaphylaxis must be available.
· Antibiotics are effective such as penicillin or erythromycin.
· Prevention; Diphtheria is effectively controlled by vaccination. Diphtheria vaccine (toxoid) usually given for children as combination with tetanus toxoid and acellular pertussis. Triple vaccine often abbreviated as DTaP
Listeria monocytogenes :
Important properties:
· Gram-positive rods arranged in V- or L-shape.
· They do not form spores.
· Catalase positive.
· Beta-hemolysis.
· Display tumbling motility.
· Grow facultative.
Habitat and transmission:
L.monocytogenes is only species that infects humans. The infection usually transmitted by ingestion of contaminated food (milk, cream, cheese, poultry). Because the organism has ability to grow at 4C, thus refrigeration does not reliably suppress its growth in food, but enhancement of growth at cold condition.
The bacteria also found in intestine of healthy persons and in vagina of asymptomatic women. The transmission of organism from person to person can occur across placenta or during delivery, and nosocomial transmission by hospital workers.
Virulence factors :
· Internalin(cell wall surface proteins) facilitates binding and endocytosis into epithelial cells and macrophages.
· Pore-forming toxin called listeriolysin-O, allows organism to escape from the endosome.
· Phospholipase-C also mediate the passage of bacteria directly to neighboring cells.
· Listeria produces siderophore to obtain iron from transferring.
L.monocytogenes has 13 serotypes. The most human infections (more than 90%) caused by serotype 1a, 1b, and 4b.
Pathogenesis:
Pathogenesis is dependent on:
· Ability of organism to ovoid immune system by spreading within the cells or between the cells.
· Movement of organism from cell to cell by uses of cellular actin and contractile system, a filament of actin that contract and propel the bacteria through the membrane of one human cell into another.
The organism is facultative intracellular parasite. The bacteria invade mononuclear phagocytic cells. The organism attaches to and enters a variety of mammalian cells, by phagocytosis(endocytosis).
Internalin of bacteria interacts with receptor (E-cadherin) on epithelium cells, promoting phagocytosis into epithelium cells. After phagocytosis , bacteria is enclosed in phagolysosome, where low pH activates the bacteria to produce listeriolysin-O(membrane-damaging toxin). This enzyme lyses membrane of phagolysosome , and allows bacteria to escape into cytoplasm of epithelium cell. It grows in cytosol and stimulates changes in cell function that facilitate its direct passage from cell to cell.
The bacteria in cytoplasm induce reorganization of cellular actin, forming (creating) elongated protrusions a comet-like tail(filopods). The complex appears to propel the organism through pseudopods in contact with adjacent cell, then the listeria is released and the cycle begins again. Phospholipase also mediate the passage of bacteria directly to neighboring cells and allowing avoidance of intracellular milieu (including cells of immune system).
Pathogenesis of Shigella and Rickettsia is same mechanism of Listeria.
Clinical features
L.monocytogenes cause meningitis and sepsis in newborns or fetuses, and in immunocompromised adults. It also causes gastroenteritis.
1-15% of healthy humans are asymptomatic intestinal carriers. The infection in pregnant women is typically asymptomatic(vaginal colonization) or the mother may has flu-like illness.
Infection during pregnancy (usually in third trimester) can cause abortion, premature delivery, or cause sepsis during peripartum period. Newborns infected can have acute meningitis 1-4 weeks later.
Perinatal listeriosis causes infantiseptica. Early-onset syndrome; intrauterine infection(sepsis) and death before or after delivery, and late-onset syndrome ;cause development of meningitis between birth and third week of life and it has significant mortality rate. The infection is most common in fetuses or newborns.
In adults, listeriosis is most common as meningoencephalitis in immunocompromised adults, such as elderly , patients receiving corticosteroids, renal transplant patients or patient with cancer.
Listeria also cause outbreak of febrile gastroenteritis which usually associated with ingestion of contaminated dairy products.
Treatment:
· Antibiotics are use for treatment of listeriosis, including ampicillin and erythromycin.
· Prevention of infection by good food handling and cooking.