Antibiotics Explained: Types, Uses, and Resistance

Antibiotics are drugs that kill or slow the growth of certain types of bacteria, and are considered broadly as antimicrobials. Antimicrobials also include antivirals, antifungals, and antiparasitic drugs. We derive antibiotics from microorganisms, like bacteria or fungi.

We use antibiotics to treat bacterial infections. Single-cell organisms are normal and natural, but can overgrow, causing infections, with unpleasant signs and symptoms in humans.

A microbe that can cause infection, and actively tries to do so at every opportunity, is called a pathogen. Many bacteria are harmless, but different strains of the same species can prove more virulent and troublesome than others. Strain is everything.

Ways in which antibiotics work

Bactericidal (kills)
Bacteriostatic (stops multiplying)

Antibiotics may interrupt the life process of a bacteria, like disrupting its ability to use glucose for energy or to build its cell wall. If this process is interrupted, it kills the bacteria and is bactericidal. Broad-spectrum antibiotics work against a wide range of bacteria, while narrow-spectrum antibiotics only work on a few.

Antibiotic resistance

Antibiotic resistance occurs when a bacteria modifies itself over time to avoid being killed by certain antibiotics. The genetic code for this resistance gets passed around, and thus a microbe can be ‘born’ naturally resistant to some bacteria.

Antibiotic resistance is occurring at a rapid pace with overuse of antibiotics by the population, driven by doctor prescribing. This is a very serious problem, since once we’ve used up all of our antibiotic options, we can’t kill the bacteria and can get very sick or have ongoing infections.

Classes of antibiotics

There are several major classes of antibiotics based on their chemical structure.

Beta-Lactams (includes sub-classes of penicillins and cephalosporins)
Macrolides
Fluoroquinolones
Tetracyclines
Aminoglycosides

Penicillins

Oldest antibiotics
Bicyclic penam compounds, share chemical structure with cephalosporins
Usually bactericidal (kills) – inhibit formation of bacterial cell wall
Treats skin infections, dental infections, ear infections, respiratory tract infections, urinary tract infections, gonorrhoea
Side-effects include diarrhoea, nausea, vomiting, upset digestion
Generally considered very safe with minimal toxicity
Rarely may cause immediate or delayed allergic reaction with skin rash, fever, anaphylaxis, severe hypersensitivity
Pregnancy category B

There are four types of penicillins:

Natural penicillins – active against gram-positive streptococci, staphylococci, some gram-negative bacteria like meningococcus
Penicillinse-resistant penicillins (methicillin, oxacillin) – active against beta-lactamase-producing bacteria, inactivates most penicillin antibiotics
Aminopenicillins (ampicillin, amoxicillin) – active agaisnt a wider range of bacteria, better oral absorption than other penicillins
Extended-spectrum penicillins (mezlocillin, piperacillin, ticarcillin)

Cephalosporins

Identical mechanism of action to penicillins, but chemical structure differs
Has beta-lactam ring that inhibits synthesis of bacterial cell wall, so bactericidal (kills)
Derived from cephalosporin C, produced by Cephalosporium acremonium
Treats pneumonia, strep throat, staph infections, tonsillitis, bronchitis, ear infections, skin infections, gonorrhoea, UTIs, bone infections
May be used as surgical prophylaxis to prevent infections
Very safe
Few side-effects, but may include diarrhoea, nausea, stomach cramps, digestive upset
Those allergic to penicillins may also have reactivity with cephalosporins and are therefore contraindicated in those with allergic reactions to penicillins
Pregnancy category B

Cephalosporins are classified by the generation they are in, with each newer generation being broader spectrum than the previous one. There are currently four generations.

#1 First generation cephalosporins

Similar to each other in spectrum, good gram-positive coverage but poor gram-negative coverage.
+ Cepahlothin
+ Cefazolin
+ Cephapirin
+ Cephalexin
+ Cefadroxil

#2 Second generation cephalosporins

Extended gram-negative spectrum plus gram-positive coverage of the first generation.
+ Cefaclor
+ Cefamandole
+ Cefonicid
+ Ceforanide
+ Cefuroxime

#3 Third generation cephalosporins

Greater gram-negative activity, but some members have lowered action against gram-positive bacteria. Can be expensive.
+ Cefcapene
+ Cefditoren
+ Cefetamet
+ Cefixime
+ Cefmenoxime
+ Cefodizime
+ Cefoperazone
+ Cefotaxime
+ Cefpimizole
+ Cefpodoxime
+ Ceftibuten
+ Ceftriaxone

#4 Fourth generation cephalosporins

Extended spectrum antibiotics with gram-positive action more like the first generation, with greater resistance to beta-lactamases. May cross blood-brain barrier, may work in meningitis.
+ Cefclidine
+ Cefepime
+ Cefluprenam
+ Cefozopran
+ Cefpirome
+ Cefquinome

Fluoroquinolones

Newest class of antibiotics
Generic name often includes ‘floxacin’
Synthetic (not bacterially derived)
Early forms named quinolones, but poorly absorbed so often used for UTIs
New forms of fluroquinolones very well absorbed orally, can be used intravenously and orally
Used to treat UTIs, skin infections, respiratory tract infections (sinusitis, pneumonia, bronchitis), lung infections like in cystic fibrosis
Bactericidal (kils) by inhibiting an enzyme DNA gyrase
Generally well tolerated and safe
Side-effects include nausea, vomiting, diarrhoea, abdominal pain
Uncommonly may cause headache, confusion, dizziness, tremor, phototoxicity, tendon rupture, convulsions

Types of fluoroquinolones include:

+ Clinafloxacin
+ Ciprofloxacin
+ Gatifloxacin
+ Levofloxacin
+ Lomefloxacin
+ Ofloxacin
+ Sparfloxacin
+ Trovafloxacin

Tetracyclines

Old class of antibiotics
Derived from Streptomyces bacteria
Bacteriostatic (hinders multiplying) agents that inhibit bacterial protein synthesis via interaction with the 30S subunit of bacterial ribosome
Broad action – works on spirochetes, atypical bacteria, rickettsia, amebic parasites
Often used to treat acne and rosacea, peptic ulcer disease, cholera, respiratory tract infections, Rocky Mountain spotted fever, Lyme disease, typhus
Common side-effects include stomach cramps, diarrhoea, nausea, vomiting, oesophageal ulcers, sore mouth, sore tongue, skin photosensitivity
May cause allergy
Do not use in children under eight, and specifically during periods of tooth development, may cause grey/yellow discolouration of actively forming teeth and deposition in growing bones
Pregnancy catergory D

Tetracyclines include:

+ Doxycycline
+ Minocycline
+ Oxytetracycline
+ Tetracycline

Macrolides

Derived from Streptomyces bacteria
Mainly bacteriostatic (hinders multiplying), targets ribosomes and prevents protein production
Newer members good for lung penetration
Treats respiratory tract infections, pharyngitis, sinusitis, bronchitis, genital infections, digestive tract infections, skin infections
Side-effects may include nausea, vomiting, abdominal discomfort and diarrhoea
Allergic reaction possible
Adverse effects possible including stomach irritation and thrombophlebitis, and in liver dysfunction
Pregnancy category B for azithromycin, erythromycin
Pregnancy category C for clarithromycin, dirithromycin, troleandomycin

Macrolide antibiotics include:

+ Azithromycin
+ Clarithromycin
+ Dirithromycin
+ Erythromycin
+ Roxithromycin
+ Troleandomycin

Aminoglycosides

Derived from species of Streptomyces
Bactericidal (kills) by binding to the 30S subunit of the bacterial ribosome, blocks protein synthesis
Treats infections by gram-negative bacteria
May be used in conjunction with penicillins or cephalosporins for better coverage
Bacteria may become resistant
Broken down quickly in stomach so must be injected
Short treatment periods
Side-effects may include damage to hearing and ear (permanent), kidney damage