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Microbiology 12 |
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Microbiology 12 |
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NOTE: INFORMATION ON THIS PAGE IS INTENDED FOR EDUCATIONAL USE ONLY. FOR MEDICAL ADVICE YOU SHOULD CONSULT WITH A PHYSICIAN.
Many of the genera of this group include important human pathogens, some of which will be discussed here. Some produce pigments and characteristically test oxidase positive.
Representative organisms:
The genus Pseudomonas includes aerobic rods with polar flagella. They typically test positive for oxidase and many species produce characteristic greenish pigments that fluoresce under UV light. Free-living forms exist in water and soil and play an important role as decomposers and take part in the nitrogen cycle. Their complex metabolism is being exploited for the purpose of bioremediation (clean up of oil spills and toxic waste, etc.). The major human pathogen is Pseudomonas aeruginosa, which is a common cause of nosocomial infections and preys on the immune-compromised.
Nosocomial infections account for 80% of deaths in burn and wound patients, the most common being due to P. aeruginosa. The crust that grows over a deep burn is called an eschar and is difficult to access with antibiotics as it lacks blood vessels. Surgical debridement of the eschar can help deliver drugs to infection sites. Infection with pseudomonas is strongly suspected if the blue-green pigment pyocyanin is present and cultures produce a sweet, fruity odor. The organism is a leading cause of lung infection in people suffering from the genetic disease called Cystic fibrosis in which the organism forms biofilms and is very difficult to treat.
Pseudomonas is notoriously resistant to many antibiotics and has also been found growing in certain disinfectants, such as "quats." The quinolone antibiotic, ciprofloxacin, is currently effective against most strains of P. aeruginosa. Strict infection control procedures must be followed in hospitals to prevent transmission of this organism, especially to wound and burn patients.
The complete genome sequence for Pseudomonas has been mapped. The website for the Pseudomonas genome project provides useful information and links about this important pathogen.
This organism was recognized as the cause of Legionnaire's disease, after a now famous outbreak of a deadly pneumonia killed 29 war veterans in a Philadelphia hotel in 1976. Legionella is a strict aerobe and a fastidious organism. Some strains are intracellular parasites of encysted amoebae including species of Acanthamoeba and Naegleria. The bacteria can also survive within biofilms, which are more resistant to chlorination and disinfectants. To date over 44 species of Legionellae have been identified and are widespread in aquatic environments. Organisms have been isolated from water cooling towers (source of the 1976 Philadelphia outbreak), shower heads, air conditioners, spas, humidifiers, vaporizers and grocery store misting devices. It is likely that this disease emerged after these devices were put into widespread use. Legionellae are typically transmitted when organisms in soil or water are aerosolized and inhaled.
The organisms are phagocytosed by macrophages but are not destroyed by phagolysosomes. Between 2-10 days, Legionnaire's disease appears as flu-like symptoms with vomiting and diarrhea followed by fluid in the lungs and pneumonia. Death can occur due to shock and kidney failure.
The organism can be identified with DNA probes and immunofluoresent antibody tests. The pneumonia is treatable with erythromycin. Penicillin is usually also given to cover the possibility of other forms of bacterial pneumonia.
Adequate levels of water chlorination are needed to control the spread of infections. Periodic cleaning of ventilation equipment also reduces the risk of nosocomial infections.
Learn more about Legionella and Legionnaire's disease. The linked website has excellent graphics, articles and updates of disease outbreaks.
This organism was named for its discover, Bordet, who along with Gengou, isolated the small, encapsulated, coccobacillus in 1906. The species name pertussis means "intensive cough" which is a hallmark of the disease known as whooping cough.
The organism is spread by droplet transmission over short distances. Humans are the only reservoir and the organism is highly contagious; over 80% of those exposed will be infected. Whooping cough primarily produces a tracheobronchitis in infants and children. Children develop prolonged, uncontrollable coughing fits. Gasping for air produces the characteristic whooping noise associated with this disease. Constant, violent coughing attacks lead to anorexia,vomiting and weight loss as well as subconjunctival hemorrhage of the eyes. The airways become clogged with a thick, sticky mucus and a child may stop breathing. Oxygen support may be necessary. Virulence is primarily due to the pertussis exotoxin but the organism also produces a tracheal cytotoxin and cell-attachment protein. This disease is NOT MILD; IT CAN BE FATAL!
B. pertussis can be identified after taking a special nasal swab for culture. The organism is slow-growing and fastidious. It does grow on a complex medium called potato-glycerol-blood agar. Identification is confirmed by florescent antibody testing.
Antibiotics do little to treat whooping cough as most of the damage has already been done by the exotoxin. However, erythromycin can be used to help reduce the spread of the disease to others.
This disease is vaccine preventable! Infants in the U.S.A. are covered in the first 6 months of life by the DTP vaccine and receive boosters in their second year and upon entering school (5 doses total). The traditional whole-cell pertussis vaccine got a bad rap when it was associated with a VERY SMALL number of serious neurological side effects, including convulsions. Read about the pertussis vaccine guidelines. Since 1996, three formulations of trivalent vaccines containing acellular pertussis (DTaP) have been approved for use in the U.S. These acellular preparations contain inactivated pertussis toxin and much less endotoxin than the whole-cell vaccine, and are considered very safe.
This organism was named Neisseria after its discover, Albert Neisser. The term, gonorrhea literally means "flow of seed" and was coined by the ancient physician Galen in 130 A.D. as he mistook the creamy pus-discharge from the genitals for a sign of virility! Gonococci are gram negative diplococcci which survive and multiply in the cytoplasm of neutrophils. The organism requires a warm, moist environment for growth. Virulent strains possess attachment pili which adhere the bacteria to the epithelium of the urogenital tract as well as to sperm. Gonococci-associated endotoxin damages the mucosa of fallopian tubes and an IgA protease destroys antibodies in body secretions
Most infected people serve as asymptomatic carriers (up to 40% of men and 60-80% of women). Regardless of whether symptoms are present or not, the infection can cause internal damage and be unwittingly transmitted to others. Gonorrhea predominately strikes sexually active young people between the ages of 20 and 24. Most cases are acquired through vaginal or anal intercourse or oral-genital sex. Discharge from the genitals in children is indicative of sexual abuse. The disease is easily transmitted. After a single exposure to an infected partner, males have a 25-30% risk of infection while females have a 50% risk! Women are at higher risk of infection as more genital surface area is exposed to infected seminal fluid.
Gonorrhea can also infect other parts of the body besides the genitals. Pharyngeal gonorrhea can result from oral-genital sex (fellatio) on an infected man. This can be mistaken for strep throat. This form of gonorrhea, as well as rectal gonorrhea, occurs with higher frequency in sexually active gay males.
The eyes can become infected after inadvertent contact with infected secretions. Newborns can acquire eye infections by passage through the birth canal of an infected mother. Although this infection can cause blindness, it has become rare because the eyes of newborns are routinely treated with erythromycin.
Symptoms of gonorrhea
Click here for more information and images of gonorrhea
In men, symptoms usually appear several days after infection and typically involves the urethra. A pus like discharge and painful urination occur and may become chronic if untreated. Symptomatic males often experience tenderness of the inguinal lymph nodes. The cervix is the most common infection site in women, often accompanied with a yellowish, pus like discharge. If untreated, or not treated early, gonorrhea can harm internal reproductive organs. In men, the epididymis can become inflamed and lead to fertility problems. Women can develop a serious condition known as pelvic inflammatory disease (PID). Symptoms include abdominal pain, vaginal discharge, pain on intercourse and irregular menstrual periods. This condition may also be asymptomatic. Untreated PID can scar the fallopian tubes, leading to infertility and ectopic pregnancies. Over 100,000 women in the U.S. become infertile due to PID annually.
Gonorrhea is diagnosed by clinical exam and by culture of organisms taken from a genital discharge.
Treatment
Gonorrhea is effectively treated with antibiotics. Gonococci have developed widespread resistance to penicillin (which was once the drug of choice) and produce B-lactamases. The CDC currently recommends a single I.M. dose of ceftriaxone to treat gonorrhea. Notably,many people with gonorrhea are coinfected with Chlamydia; up to 25% of males and 50% of females. For this reason people with gonorrhea are usually also treated for chlamydia with a week long course of doxycycline. Another disturbing fact is that quinolines have been widely used in the Philippines to treat NG and drug resistant strains of the bacteria have been reported. The appearance of these strains may make it impossible to use quinolones for NG in this country in the near future.
This gram negative diplococcus is the most common cause of epidemics of bacterial meningitis. The disease is highly contagious and is spread by droplet transmission. 15-30% of the general population act as asymptomatic carriers, harboring the organism in the nasopharynx. The spread of disease is thought to be facilitated by factors such as overcrowding and lack of immunity. The disease most often strikes youth and young adults. A meningitis belt occurs across central Africa which sees many thousands of deaths during dry season from Jan to June. Drying of mucus membranes, crowded conditions, coinciding with the flu season make people more vulnerable to infection in this part of the world. Early tracking and warning of epidemics are needed to provide people enough time to be vaccinated.
The organism colonizes the nasopharyngeal mucus membranes by means of adhesins that protect the organisms from mucociliary defenses. Like N. gonorrhoeae, meningococci produce IgA proteases, which destroy mucosal antibodies. Meningococci also steal host iron, normally bound by transferrin and lactoferrin. The most common route of dissemination to the CNS is the bloodstream. The organism is protected from phagocytes by a polysaccharide capsule.
The illness usually begins as a sore throat and may resemble a mild cold, followed with the sudden onset of a severe, throbbing headache, fever and marked stiffness of the head and back. Petechial hemorrhages (non-raised purplish, round spots) containing organisms appear under the skin. Coma and swelling of the brain follow. Meningococci can disseminate and cause a complication known as Waterhouse-Friderichsen syndrome. Death can occur within hours from endotoxic shock Gram negative sepsis may also require amputation of affected extremities.
Diagnosis is made by isolation of meningococcci from petechiae or CSF. The organism grows well on enriched Chocolate (heated blood) agar in an atmosphere of 5-10% C02.
If untreated the disease has a high mortality rate of up to 90%. Penicillin is the drug of choice to treat meningococcal meningitis. Ampicillin and Cephalosporins are also used. The antibiotic rifampin is used to eliminate the carrier state and to prevent outbreaks.
There are 12 serotypes of meningococci, with A, B and C being the most prevalent. A polysaccharide capsular vaccine is available against types A, C, Y and W but not the most common type B. The vaccine is not immunogenic in young infants.
For more information about the different causes and symptoms of meningitis go to the Meningitis Foundation of America