History

 

Bacteria are one of the oldest forms of life on earth, thought to have originated 3.22 billion years ago (Homann et al., 2018). There is an estimated one trillion species in existence, although most of these have not yet been discovered. They usually exist as single cell organisms (prokaryotic), however they form groups called colonies which are visible to the human eye.

 

Structure

 

Bacterial cells do not contain a nucleus- they contain loose genetic material housed in a cell membrane and cell wall. There are variations within the composition of these outer cell structures, therefore classifying bacteria into two groups: Gram-positive and Gram-negative (see Figure 1).

 

Gram-positive bacteria have a thin inner plasma membrane (composed of lipids) and a thick peptidoglycan layer (composed of carbohydrates and proteins).

 

Gram-negative bacteria have a thin inner plasma membrane, a thin peptidoglycan layer, and then another thin outer plasma membrane (Silhavy et al. 2010).

 

Gram-positive, Gram-negative and broad-spectrum antibiotics

 

Some antibiotics are target specific to Gram-positive or Gram-negative. For example, penicillin targets proteins within the peptidoglycan layer. This makes penicillin effective against gram-positive bacteria, because they contain a thick peptidoglycan layer which is also the outer most layer. Penicillin however is not very effective against gram-negative bacteria, because of the outer membrane protecting the peptidoglycan layer. Therefore, Gram-negative bacteria are already resistant to penicillin due to their outer structure.

 

When antibiotics target both Gram-positive and Gram-negative bacteria, they are known as broad-spectrum antibiotics.