Antibiotics (part 1)

Okay, guys.  I’m gonna rep microbiology for a minute so we can talk about antibiotics. 

Before we can talk about antibiotics, we need to talk about bacteria. 

In the late 1800’s, a Danish scientist developed an eponymous test called the Gram stain.  This test allowed him to determine the nature of the bacterial cell wall in medical samples.  Speaking very generally, this test, which requires four very cheap reagents and a microscope, turns bacteria on the slide either purple or pink.  If the bacteria turn purple, they are called Gram positive.  If the bacteria turn pink, they are called Gram negative.

Another important thing about bacteria is the way they are shaped.  They are often round (cocci) or rod-shaped (bacilli.)  There are some other shapes, like spirochetes (the organism that causes Lyme Disease is a spirochete), but for simplicity, let’s just deal with rods and cocci for now.

Bacteria are also aerobic (use oxygen for energy) or anaerobic (prefer environments that are low on oxygen.)  This can affect which types of antibiotics will work on them. 

So, let’s say you are at a hospital and have a blood infection.  They will draw blood from you.  Some of this blood will be used for a Gram stain.  In this test, a drop of the blood will be put on the slide and dried.  It will then be Gram stained, which is a very quick procedure.  The slide will then be viewed under a microscope.

So if it looks like this:

It is called a gram positive cocci (GPC.)  Gram positive cocci include S. aureus (including MRSA), Enterococcus faecalis, and Streptococcus pneumoniae.  S. aureus causes several types of infections, including skin and bloodstream infections. E. faecalis also causes a variety of infections including meningitis and UTI.  Strep pneumo causes pneumonia and other infections.

If it looks like this:

It is a gram negative rod (GNR.)  Gram negative rods include E. coli, K. pneumoniae and Pseudomonas aeruginosa.  P. aeruginosa is a very dangerous organism that is resistant to many types of antibiotics.  E. coli causes a lot of issues, including food poisoning and urinary tract infections.  K. pneumoniae causes pneumonia and other infections.

All gram positives are not necessarily cocci, and all gram negatives are not necessarily rods.

Knowing the shape and Gram stain result of an organism allows the doctor to identify what types of antibiotics are likely to work.  The reason for this is that the Gram stain tells us what kind of cell wall the bacteria have, and some antibiotics work based on the cell wall structure.    

Antibiotics are medications that inhibit the growth of or kill microorganisms, specifically bacteria and protozoans (parasites.)  Medications that kill fungi are called antimycotics or antifungals, but are sometimes generally referred to as antibiotics as well.  There are also antiviral medications, but antivirals are not usually considered antibiotics because viruses are not technically alive. 

Penicillin antibiotics include a plethora of similarly named medications, including amoxicillin, ampicillin, dicloxacillin, oxacillin and so on.  They are beta-lactams, so an allergy to penicillin often extends to cephalosporins.  Penicillin allergies are often very severe due to activation of the four major allergic pathways by the penicillin molecule.  Penicillin was the first known antibiotic and treats lots of Gram positive and spirochete infections.  There are several medications that pair penicillin with another component like sulbactam and clavulanate.  In these combinations, the second drug interferes with bacterial resistance mechanisms.

First generation cephalosporins are effective against only Gram positive bacteria.  This is called “narrow spectrum action.”  They stop bacteria from making their cell walls.  These include drugs like cephalexin.  All cephalosporins are beta-lactam antibiotics, the name for antibiotics that contain a certain structure.

Second generation cephalosporins are drugs like cefprozil and cefuroxime.  They treat gram negative infections and some gram positive infections. 

Third generation cephalosporins treat most gram negative infections but not those caused by Pseudomonas aeruginosa. 

Cefepime is a fourth generation cephalosporin.  It treats Pseudomonas aeruginosa infections as well as other gram negative infections. 

Fifth generation cephalosporins treat MRSA (methicillin-resistant Staph aureus) infections.  They should only be used for that purpose.

Glycopeptides affect gram positive cell walls.  They treat aerobic and anaerobic Gram positive infections including MRSA.  Vancomycin is a glycopeptide.  They also treat C. difficile (C Diff.)

Lincosomides like clindamycin are mostly used for serious Gram positive infections in people who are allergic to penicillin.  Using them can make you more likely to develop C Diff (I’ll explain why that is in my next post.)

Aminoglycosides include gentamicin and neomycin.  These two drugs kill aerobic, gram negative bacteria like E. coli and Ps. aeruginosa.  Streptomycin treats tuberculosis, which is caused by a type of bacteria called Mycobacteria.  They stop the bacteria from making proteins.

Macrolides include azithromycin, clarithromycin, erythromycin and a bunch of others.  They treat Gram positive infections including strep throat, as well as spirochete infections like syphilis and Lyme Disease.

Fluoroquinolones are the subject of hot debate in recent years.  I’m going to do a post on just fluoroquinolones.  These are drugs like ciprofloxacin and levofloxacin.  They interfere with DNA replication.  They are very broad spectrum and can treat Pseudomonas infections.

Sulfonamides prevent bacteria from metabolizing folic acid.  They can treat gram positive and gram negative organisms.  They are used often for UTI and burns.

Tetracyclines interfere with protein synthesis.  They can treat spirochete infections, Mycoplasma infections and malaria.  (Malaria is not actually caused bacteria or protozoa, but by a protest.)  They can cause severe birth defects and can affect bone growth. 

Carbapenems kill both Gram positive and negative bacteria.  This is called “broad spectrum action.”  These drugs are often reserved for infections that have not responded to other antibiotics.  Meropenem is an example.  MRSA are resistant to these drugs.

Daptomycin is a very unusual antibiotic that is active against Gram positive bacteria. 

Aztreonam is active against gram negative bacteria.

Nitrofurantoin is a nitrofuran.  It is used only to treat urinary tract infections.

Linezolid is often the drug of last resort for Staph infections that are resistant to methicillin and vancomycin.  It is a very serious medication and can cause low platelet count and neurologic pain.  People who are prescribed this medication usually have no other option.

Bacitracin and polymyxin B are polypeptides.  The first interferes with cell membrane building and the second affects Gram negative cell membranes.  They are usually applied to the eye or ear, or inhaled.  When injected or infused, they carry the risk of kidney and nerve damage.

There are a lot of other antibiotics, like metronidazole and fosfomycin.  These generally treat very specific types of infections. 

Fun fact: antibiotics that end in –micin or –mycin are not necessarily from the same drug class.  This suffix indicates that it was derived from or first found in fungi. 

Polymyxin B, colistin and vancomycin are not recommended for mast cell patients.  Like food, antibiotic triggers are very individual.

Tune in tomorrow for part 2 of my antibiotic spiel, in which I tell you all about antibiotic resistance, how big of a problem it’s becoming and why culturing an infection is so important.

Image references:

courses.ahc.umn.edu

www.microbeworld.org