Bacteria are single celled organisms. They come in many shapes and sizes. Most bacteria are not harmful however some are pathogenic and cause infectious diseases. These pathogenic bacteria are the most relevant to learning medicine.

They can be categorised into aerobic and anaerobic, gram positive and gram negative and atypical bacteria. Learning where bacteria fall within these categories helps you work out which antibiotics will be effective against them.

Aerobic bacteria require oxygen whereas anaerobic bacteria do not. Gram positive bacteria have a thick peptidoglycan cell wall that stains with crystal violet stain. Gram negative bacteria don’t have this thick peptidoglycan cell wall and don’t stain with crystal violet stain but will stain with other stains. Atypical bacteria cannot be stained or cultured in the normal way.

Bacteria can also be classified based on their shapes. Rod shaped bacteria are called bacilli and circular shaped bacteria are called cocci.


Basic Anatomy and Physiology


There are some key components to learn about as they are the target of antibiotic treatment:

The cell wall is a structure that surrounds the outer cell membrane and is found on gram positive bacteria.

Nucleic acid is essential component of bacterial DNA.

Ribosomes are where bacteria proteins are synthesised within the bacterial cell.

Folic acid is essential for synthesis and regulation of DNA within the bacteria. Folic acid cannot be created or directly imported into the cell and requires a chain of intermediates to get in to the cell. This chain starts with PABA, which is directly absorbed in to the cell across the cell membrane. PABA is then converted to DHFA which is converted inside the cell to THFA then folic acid.


Gram Stain

A gram stain is used as a quick way to check a sample under the microscope to look for bacteria. It involves two main steps:

Add a crystal violet stain which binds to molecules in the thick peptidoglycan cell wall in gram positive bacteria turning them violet.

Then add a counterstain (such as safranin) which binds to the cell membrane in bacteria that don’t have a cell wall (gram negative bacteria) turning them red/pink.


Gram-Positive Cocci

  • Staphylococcus
  • Streptococcus
  • Enterococcus

Gram-Positive Rods

Use the mnemonic “corney Mike’s list of basic cars”:

  • Corney – Corneybacteria
  • Mike’s – Mycobacteria
  • List of – Listeria
  • Basic – Bacillus
  • Cars – Nocardia


Gram Positive Anaerobes

Use the mnemonic “CLAP”:

  • C Clostridium
  • LLactobacillus
  • AActinomyces
  • PPropionibacterium


Gram Negative Bacteria

If the bacteria is not listed above then it is probably gram negative. Common gram negative organisms are:

  • Neisseria meningitis
  • Neisseria gonorrhoea
  • Haemophilia influenza
  • E. coli
  • Klebsiella
  • Pseudomonas aeruginosa
  • Moraxella catarrhalis


Atypical Bacteria

The definition of atypical bacteria is that they cannot be cultured in the normal way or detected using a gram stain. Atypical bacteria are most often implicated in pneumonia.

The atypical bacteria that cause atypical pneumonia can be remembered using the mnemonic “legions of psittaci MCQs”:

  • Legions – Legionella pneumophila
  • Psittaci – Chlamydia psittaci
  • M – Mycoplasma pneumoniae
  • C – Chlamydydophila pneumoniae
  • Qs – Q fever (coxiella burneti)


Methicillin-Resistant Staphylococcus Aureus (MRSA)

MRSA refers to staphylococcus aureus bacteria that have become resistant to beta-lactam antibiotics such as penicillins, cephalosporins and carbapenems.

They are a problem in healthcare settings where antibiotics are commonly used. Think about MRSA in patients that have had hospital admissions or come from a nursing home or other healthcare institution.

People are often colonised with MRSA bacteria and have them living harmlessly on their skin and respiratory tract. If these bacteria become part of an infection they can be difficult to treat. Patients being admitted for surgery or treatment are screened for MRSA infection by taking nasal and groin swabs so that extra measures can be taken to try to eradicate them and stop their spread. Eradication usually involves a combination of chlorhexidine body washes and antibacterial nasal creams.

Antibiotic treatment options for MRSA are:

  • Doxycycline
  • Clindamycin
  • Vancomycin
  • Teicoplanin
  • Linezolid


Extended Spectrum Beta Lactamase bacteria (ESBLs)

ESBLs are bacteria that have developed resistance to beta-lactam antibiotics. They produce beta lactamase enzymes that destroy the beta-lactam ring on the antibiotic. They can be resistant to a very broad range of antibiotics.

ESBLs tend to be e. coli or klebsiella and typically cause urinary tract infections but can also cause other infections such as pneumonia.

They are usually sensitive to carbapenems such as meropenem or imipenem.


Last updated February 2019