Microbiology: Culture + Sensitivity ✨

Culture + sensitivity is one of those phrases you hear most days in practice, but it can feel confusing.

You might see a result come back + you're unsure of what it all means.

But the basic idea is: 

• culture tells us what grew
• sensitivity tells us what may work against it

✨ What does culture mean?

Culture means the sample is placed in conditions that allow microorganisms to grow.

If bacteria or fungi are present, the lab may be able to identify what has grown.

A culture result may tell you:

• whether anything grew
• what organism was found 
• whether growth was light, moderate or heavy
• whether there was mixed growth
• whether the sample may have been contaminated

This helps the vet understand what organism may be involved in the patient's infection.

✨ What does sensitivity mean?

Sensitivity testing looks at which antimicrobials are likely to be effective against the organism that grew.

The lab tests the organism against different antibiotics.

The result may show whether the organism is:

• sensitive
• resistant
• sometimes intermediate, depending on the lab reporting system

This helps guide antibiotic choice.

It does not mean the nurse is choosing the antibiotic, but understanding the result helps you follow the case + understand why certain treatment decisions are made.

✨ Sensitive vs. resistant

These terms are important.

Sensitive - if an organism is sensitive to an antibiotic, it means that antibiotic is likely to be effective against it.

Resistant - if an organism is resistant, it means that antibiotic is unlikely to work properly against that organism.

This is why culture + sensitivity is so helpful in cases that are:

• recurrent
• severe
• not responding to treatment
• unusual
• linked to surgical site infection
• suspected to involve resistant bacteria

✨ Why culture results need context

A culture result should never be read completely on its own.

It needs to be interpreted alongside:

• the patient
• the clinical signs
• the sample site
• how the sample was collected
• whether the sample could have been contaminated 
• the patient's treatment history

For example, bacteria found on a superficial wound swab may not tell us as much as a clean sample taken from a deeper tissue.

✨ Light, moderate + heavy growth

Sometimes culture reports describe growth as light, moderate or heavy.

This gives an idea of how much organism grew from the sample.

But again, it needs context.

Heavy growth may be more significant, especially if it matches the clinical signs.

Light growth may still matter in some cases, but it may also reflect contamination or colonisation depending on the sample.

✨ Gram-positive + gram-negative bacteria

You may also see bacteria described as gram-positive or gram-negative.

This comes from a staining technique, which helps classify bacteria based on their cell wall structure.

Gram-positive bacteria:

• these usually stain purple 
• common examples include: staphylococcus, streptococcus

Gram-negative bacteria:

• these usually stain pink or red
• common examples include: salmonella, leptospira

You do not need to memorise every organism, but recognising these terms should help your confidence.

✨ Why sensitivity matters

Sensitivity testing supports responsible antibiotic use.

Antibiotics should not be chosen randomly, especially in cases where infection is recurrent, severe or not improving.

Using sensitivity results can help:

• choose a more appropriate antibiotic
• avoid unnecessary antibiotic use
• reduce treatment failure
• support better patient outcomes
• help protect antibiotics for future use

This links closely to antimicrobial resistance.

✨ What is antimicrobial resistance?

Antimicrobial resistance happens when microorganisms change in a way that means medicines that used to work against them are no longer effective.

Essentially, the bacteria learn how to survive the antibiotic!

This can make infections harder to treat.

Responsible antibiotic use matters because every unnecessary or inappropriate use can add pressure for resistance to develop.

This is why culture and sensitivity is so important - it helps us move away form guessing + towards treatment that is more targeted.

✨ The nurse's role in responsible antibiotic use

Veterinary nurses do not prescribe antibiotics, but we still play an important role.

Nurses can support responsible use by:

• collecting + handling samples properly 
• following infection control protocols
• monitoring whether the patient is improving
• noticing side effects or deterioration 
• communicating changes clearly to the vet 
• supporting client education around completing treatment correctly
• helping reduce cross-contamination in practice 

✨ Key takeways

• Culture tells us what grew
• Sensitivity tells us what antibiotics may work
• Results need to be interpreted alongside the patient
• Sensitive means likely to respond
• Resistant means unlikely to work
• Culture and sensitivity supports responsible antibiotic use
• Nurses help by collecting good samples, monitoring patients and supporting infection control

 

I really hope this helps! 

Thank you for taking the time to read this week's blog, Jade 🤍