(From Micro, 9 Oct 2000, by Brian Buschman)
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After studying the gram positive cocci and rods that produce spores here are the two classes of gram positive rods that do not produce spores. They are rather simple since you only have one bug for each set.
C. diphtheriae causes diphtheria by colonizing the pharynx and forming a pseudomembrane of fibrin, leukocytes, necrotic epithelium and bacterial cells. From the pharynx C. diphtheriae releases exotoxin that attacks the neural system and the heart. It works because of two subunits. The A subunit enters the cell to do the work. The B subunit of the exotoxin helps the A subunit get in. The work it actually does is to block human ribosomes so it works like an antibiotic against humans.
Diagnosis is based on plating on potassium tellurite where C. diphtheria forms gray/black blobs. You can also use Loeffler’s coagulated blood serum media which gives rods that you see after staining with methylene blue.
Treatment is with penicillin or erythromycin and by giving the patient another DPT vaccine. Just having the disease might not convey immunity which is the reason for the booster.
L. monocytogenes is our last gram positive rod which is also b-hemolytic, catalaze positive and makes an endotoxin.
It only infects neonates and immunosupressed people. The reason for that is in immunocomponent people L. monocytogenes is engulfed and killed by immune cells. In neonates and the immunosupressed it is engulfed but then lives inside the host cell. It is a facultative intracellular organism. It then causes meningitis.
You treat L. monocytogenes with ampicillin or trimethoprim-sulfamethoxazole.
The Neisseria are gram-negative diplococci.
N. meningitidis is a major cause of meningitis. It’s morphology includes:
1) Capsule that protects it. There are many types of capsule groups A, B, C, X, Y, Z and W-135.
2) LPS endotoxin which causes destruction of blood vessels. This allows it to get into the skin and cause it’s classis rash “petechiae.” It also allows it to get through into the adrenal which causes the septic shock talked about later.
3) IgA protease.
4) The ability to extract Fe2+ from transferrin.
It primarily only infects people when they are only 6-24 months old and in army recruits. It gets the little ones because they are given immunity from mom but the maternal IgG is usually gone after about 6 months. Many people have a strain in their system which they become immune to but in basic training the guys tend to give their strains to each other.
N. meningitis rarely gets into the system from the pharynx but when it does hit the blood stream three things can happen:
1) Meningococcemia is meningococci in the blood which will break holes in the cutaneous vessels and cause the “petechial” rash.
2) Fulminant meningococcemia (septic-shock) is the way the meningococci break through the walls of the blood vessels. They do it to get to the adrenal gland and cause adrenal insufficiency. That in turn causes low BV which leads to hypotension, tachycardia, DIC and coma.
3) Meningitis is meningococcal disease. You will often see classic meningitis with petechiae. That is the tip-off.
You can grow Neisseria on chocolate agar but to diagnosis it you plate it on Thayer-Martin VCM media. (V for vancomycin which kills gram-positives, C for colistin to kill gram-negatives except Neisseria and Nystatin to ill fungi.) It grows best with a high [CO2].
N. meningitidis can metabolize maltose while N. gonorrhoeae cannot.
Treat with penicillin G or ceftriaxone. Close contacts are treated with rifampin to be safe.
N. gonorrhoeae’s virulence factors include:
1) Pili that adhere to the host cells but also have rapidly changing genes that mutate the pili so they cannot be targets for Abs or vaccines.
2) Protein II is an outer membrane protein associated with adherence to host cells.
In men gonorrhea causes urethral infection and can infect all sorts of male tubes. “It hurts when I pee” can easily be cured with ceftriaxone.
In women it does less to the urethra but is brutal on the genital tract. It causes inflammation of all the parts. It will cause pain with sex, PID and all. If you have a young woman with left sided appendicitis it’s probably PID. The infection’s spread is faster with IUDs or menstration.
PID can lead to:
1) Sterility from the scaring
2) Extopic pregenancy from the scaring
3) Abscesses
4) Peritonitis
5) Perihepatitis is an infection of the stuff around the liver. It causes UR quadrant tenderness.
More stuff for either gender:
1) On rare occasion gonorrhoeae can invade the bloodstream and cause symptoms all over.
2) Septic arthritis is an inflammatory disease of joints.
Infants can become infected from mom during delivery. It causes opthalmia neonatorium which is the reason for treatment with erythromycin or silver nitrate.
Diagnose once again with Thayer-Martin VCM media for Neisseria and look for maltose metabolism. N. meningitidis will metabolize maltose and N. gonorrhoeae cannot.
Treat gonorrhea with ceftriaxone which is a third generation cephalosporin. Use it because of MDR. This also will be effective in treating syphilis. Administer it with doxycycline or azithromycin to treat Chlamydia trachomatis because the two diseases often appear together.
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