Impetigo and Cellulitis: How to Tell Them Apart and Choose the Right Antibiotic

Two kids come home from school with red, crusty sores around their noses. A parent notices their leg is swollen, hot, and bright red after a small cut healed over. Both scenarios are common - and both could be bacterial skin infections. But impetigo and cellulitis are not the same. Mixing them up can lead to wrong treatment, longer illness, or even serious complications. Knowing the difference isn’t just helpful - it’s necessary.

What Impetigo Looks Like (and How It Spreads)

Impetigo is the classic "school sores" infection. It’s most common in children aged 2 to 5, but adults can get it too - especially if they have eczema, insect bites, or minor cuts. The infection starts small: a red spot or blister, often near the nose or mouth. Within hours, it bursts, oozes, and forms a thick, golden-yellow crust. That’s the hallmark of nonbullous impetigo, which makes up about 70% of cases.

The other type, bullous impetigo, is less common. It shows up as larger, fluid-filled blisters (2-5 cm wide) that don’t burst right away. When they do, they leave behind a ring-like border. These blisters are filled with clear or yellow fluid and are usually caused by Staphylococcus aureus releasing a toxin that weakens the skin layers.

What makes impetigo so tricky is how easily it spreads. One child scratching a sore can transfer the bacteria to their hands, then to toys, towels, or another child’s skin. It doesn’t even need broken skin - Staphylococcus aureus can invade intact skin, especially in warm, humid weather. That’s why outbreaks happen fast in daycare centers and schools. Health guidelines say kids should stay home until they’ve been on antibiotics for at least 24 hours.

What Cellulitis Feels Like (and Why It’s More Dangerous)

Cellulitis is deeper. It doesn’t just sit on the surface - it invades the dermis and fat layer under the skin. You don’t see crusts or blisters. Instead, you feel it: a swollen, warm, painful patch of skin that’s red and spreading. The edges are blurry, not sharp. It often shows up on the legs, but can appear anywhere - arms, face, even around the eyes.

Unlike impetigo, cellulitis is rarely contagious person-to-person. But it’s more dangerous. If left untreated, it can spread to the bloodstream, causing sepsis. People with diabetes, poor circulation, or weakened immune systems are at higher risk. Even a tiny scratch from gardening or a pet’s nail can become a portal for bacteria.

There’s a related condition called erysipelas - often confused with cellulitis. Erysipelas has a sharp, raised border and is almost always caused by Streptococcus. It’s more common on the face and tends to come on faster. But both need urgent attention.

What Bacteria Are Behind These Infections?

Both infections are caused by the same two main bacteria: Staphylococcus aureus and Streptococcus pyogenes. But their roles differ.

• Impetigo: Mostly Staphylococcus aureus (especially in bullous type). Nonbullous impetigo often involves both Staph and Strep.
• Cellulitis: Primarily Streptococcus, especially in children. In adults, Staphylococcus aureus is more common - and increasingly, MRSA (methicillin-resistant Staphylococcus aureus).

MRSA is the big worry. It doesn’t respond to common antibiotics like flucloxacillin or amoxicillin. In hospitals and communities, MRSA accounts for up to 40% of skin infections in some areas. That’s why doctors now check for risk factors: recent hospital stays, IV drug use, or living in crowded places. If you’ve had a skin infection that didn’t improve with standard treatment, MRSA might be the culprit.

Antibiotic Choices: UK vs. France vs. Belgium

There’s no global standard. Antibiotic guidelines vary by country - and even by local resistance patterns.

In the UK, flucloxacillin is the go-to for both impetigo and cellulitis. It’s a penicillin-type antibiotic that works well against Staphylococcus. For mild impetigo, a topical cream like mupirocin (applied three times a day for 7 days) is often enough. If the infection is widespread, oral flucloxacillin for 7-10 days is standard.

In France, doctors lean toward amoxicillin for cellulitis and pristinamycin or amoxicillin-clavulanate for impetigo. Why? They’ve seen more resistance to flucloxacillin locally and adjust accordingly.

Belgium doesn’t have national guidelines - doctors pick based on experience. But across all three countries, flucloxacillin use for impetigo has climbed sharply in the last decade, from around 55% to over 80% in the UK.

For MRSA cases, doctors switch to clindamycin, doxycycline, or trimethoprim-sulfamethoxazole. In severe cellulitis - especially with fever, chills, or spreading fast - hospitalization and IV antibiotics like vancomycin may be needed.

When to Use Topical vs. Oral Antibiotics

Not every case needs pills.

Topical antibiotics (like mupirocin or fusidic acid) work well for small, isolated patches of impetigo - say, one or two sores on the face. They’re safe, low-cost, and reduce the risk of antibiotic resistance. But if the infection covers more than a few square centimeters, or if there are multiple sores, oral antibiotics are better. Topical treatments don’t reach deep enough for cellulitis.

Oral antibiotics are the standard for cellulitis. Even if the red area looks small, it’s growing beneath the skin. A 5-14 day course is typical. If symptoms don’t improve in 48 hours, you need to go back. That’s not a sign the antibiotic isn’t working - it’s a sign you might need a different one, or that the infection is deeper than it looks.

What Happens If You Wait Too Long?

Delaying treatment is risky.

With impetigo, waiting more than 72 hours increases the chance of spreading to others - and sometimes leads to kidney inflammation (post-streptococcal glomerulonephritis), though this is rare.

With cellulitis, every hour counts. Untreated, it can lead to abscesses, tissue death (necrotizing fasciitis), or sepsis. One study found that patients who waited over 48 hours to seek care were 3 times more likely to need hospitalization.

And here’s something many don’t realize: cellulitis often comes back. About 20% of people have a second episode within a year. That’s why doctors check for underlying causes - like athlete’s foot, leg swelling, or poorly controlled diabetes.

Prevention: Simple Steps That Make a Big Difference

You can’t always avoid infection, but you can cut the risk.

• Wash cuts and scrapes right away with soap and water. Cover them with a clean bandage.
• Don’t share towels, razors, or clothing - especially if someone has a skin infection.
• Keep fingernails short. Scratching spreads bacteria.
• Treat eczema and athlete’s foot. These create openings for bacteria.
• If you have diabetes, check your feet daily. A small sore can turn into cellulitis fast.

For families with kids in school or daycare, a simple rule: if your child has a weeping, crusty rash, keep them home until they’ve had antibiotics for 24 hours. It’s not overcautious - it’s necessary.

When to See a Doctor

Not every red patch needs a prescription. But here’s when to act:

• Redness spreading quickly
• Swelling, warmth, or pain worsening
• Fever, chills, or feeling unwell
• Sores that don’t improve after 3 days of treatment
• Recurrent infections (more than twice a year)

Don’t wait for it to look "bad." If you’re unsure, get it checked. A quick visit can prevent a hospital stay.

Antibiotic Resistance Is Real - But Manageable

Overuse of antibiotics has made some infections harder to treat. But doctors aren’t just prescribing blindly anymore. In clinics across the UK and Europe, there’s a growing push for antibiotic stewardship.

That means:

• Only prescribing antibiotics when needed
• Choosing narrow-spectrum drugs first (like flucloxacillin, not broad ones like ciprofloxacin)
• Using cultures and sensitivity tests for recurrent or treatment-resistant cases

Studies show that up to 30% of skin infection prescriptions are unnecessary. But when done right - matching the drug to the bug - recovery is fast and complications drop.

The future? Faster tests to identify bacteria and resistance genes in under an hour. That’s already happening in some hospitals. In five years, we’ll likely see treatment guided by local resistance maps - not just national guidelines.