﻿ Bacteria | Smith & Nephew - New Zealand

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This information is intended for Healthcare Professionals only.





What is methicillin-resistant Staphylococcus aureaus (MRSA)?
Staphyloccus aureus which is resistant to an antibiotic called Methicillin and in most cases other Beta-lactam antibiotics is referred to as methicillin-resistant staphylococcus aureus or MRSA.

MRSA can cause problems when it gets the opportunity to enter the body. This is more likely to happen in people who are already unwell. MRSA can infect wounds - accidental wounds such as grazes and deliberate wounds such as those made from a drip or during surgery.


If bacteria are everywhere, why don't we all get ill all of the time?

Acticoat Staphylococcus image
Because intact healthy skin acts as a physical barrier:
  1. Skin pH is not conducive to pathogenic bacterial growth
  2. Skin secretes fatty acids and antibacterial polypeptides
  3. Normal skin flora help in preventing pathogenic flora from growing


However, a wound provides the ideal gateway for bacteria to enter the body…


When the skin is compromised through a wound, cut or surgical procedure, the normal flora present on the skin is able to invade the defect and colonise the soft tissue.
Normally, the immune system is able to fight off the invasion (host resistance) and keep the population at a controllable level. Host resistance, however, can be reduced by a number of systemic factors, including (but not limited to):

  • Diabetes mellitus
  • Vascular disease
  • Malnutrition
  • Immunosuppressive drugs

And when the host's resistance is low then the colonised microorganisms are able to grow at rates, which exceed the control of the immune systems and can result in infection.


If the bacteria start to colonise the wound, then naturally, the number of microorganisms in the wound increases. Bacteria divide and grow at an exponential rate, which is why it's so important to address infection as soon as it is detected.


We use the phrase ‘Bacterial Burden' to describe the level of bacteria that is found in the wound at any given time.


And we often measure the number of bacteria in Colony Forming Units per cubic centimetre (abbreviated to C.F.U.).




10 6



10 5



10 4



10 3



10 2



10 1


A colonisation level of between 104 and 105 CFU's or more will generally be associated with an infection, though lower numbers can cause it, depending on:

  1. the species of bacteria involved and their virulence factors
  2. the health of the patient.


NB: It's important to remember that each individual patient reacts to different levels of bacteria in different ways, which is why it's so important to also take into account clinical signs, such as heat and swelling around the wound.

Bacterial Burden and the contamination-infection continuum
The level of bacteria in the wound runs from contamination to infection.


Contamination Colonisation

Critical colonisation/

increased bacterial burden




- - - - - - ACTICOAT Zone - - - - -

chart showing contamination-infection
In the management of wounds such as venous leg ulcers and pressure ulcers, the essential factor is that bacteria are not replicating, and are present in such small numbers that they will not affect healing. An antimicrobial dressing would not be recommended. This refers to a wound where bacteria are present and are replicating. Bacterial colonisation may produce minor patient reactions, but will not result in an extreme response and will not affect healing. An antimicrobial dressing would not be recommended.
These terms are often used interchangeably. This can be described as a phase where the wound is not showing classic signs of infection, however, bacteria are replicating and healing is delayed. An antimicrobial dressing would be recommended.
Bacteria have invaded the tissue and are replicating. There is a systemic reaction in the host and healing is delayed. Along with other appropriate protocols an antimicrobial dressing would be recommended.

It's all about getting the right balance
A bacterial balance must be achieved in order for wound healing to commence.


Host resistance   Bacterial quantity
and virulence
  Bacterial Balance scales  
  Bacterial balance  



One factor that can greatly affect bacterial balance is the formation of biofilms .


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