Antibiotic resistance

Penicillin resistance

Resistance to penicillin is usually due to Beta lactamase enzymes.  Therefore adding a  beta-lactamase inhibitor eg clavulanate (as found in co-amoxiclav) overcomes the resistance and extends the spectrum.

An alternative resistance mechanism however is production of defective Penicillin Binding Proteins – this is the mechanism of resistance in Pneumococci.  Beta lactamase inhibitors therefore do not help.

Macrolide resistance

Variable resistance seen; sometimes effective against penicillin resistant staphylococci including some MRSA, but poor activity against Haemophilus. Variable resistance seen in streptococcus, pneumococcus. Resistant mycoplasma are rare but do exist – try cipro, else tetracycline.

Using Azithromycin probably improves efficacy in Haemophilus (more active) but pneumococci that are resistant to erythromycin (approximately 5 to 20% of strains currently) will also be resistant to azithro, and no difference in in vitro activity between the newer and old macolides against other common respiratory pathogens.

Multiresistant organisms

In theory, you should use a combination of antibiotics (if possible) to treat a multiresistant organism, to prevent resistance developing (as in TB).  However there is no good clinical data to support this, beyond TB treatment (which is slow growing, so probably different).  On the contrary, meta-analyses demonstrate no difference in clinical outcomes between the two treatment strategies (for infections with Gram-negative bacteria), but there are well-documented increased toxicities with combination therapy.

Having said that, given the greater mortality associated with delays in appropriate and effective antimicrobial treatment, starting with combination therapy in critically ill patients seems sensible.

If there is poor response to treatment, rather than simply adding a second agent, consider:

  • dose, frequency – are you achieving adequate time above MIC?  Consider prolonged antibiotic infusion strategy
  • route – give IV if not already
  • duration – extended treatment course?

[doi: 10.1128/CMR.05041-11 Clin. Microbiol. Rev. July 2012 vol. 25 no. 3 450-470]



ESBL – extended spectrum beta lactamase

Scenario 1: lower renal tract infection with ESBL producing Gram negative

Bacteriuria with urinary symptoms but minimal systemic symptoms and no fever

  • Treatment duration 3 days
  • First line – trimethoprim / nitrofurantoin  (not used in under 3 months)
  • Depending on sensitivities and age alternative options include ciprofloxacin, single dose aminoglycoside, pivmecillinam or fosfomycin

Scenario 2: upper renal tract infection with ESBL producing Gram negative

Bacteriuria with urinary symptoms and associated systems symptoms including fever

  • Treatment duration 7-10 days (as per NICE)
  • Always consult with infection management specialist
  • First line – if seriously unwell or under 3 months carbapenem  + consider one dose of aminoglycoside
  • Alternative: tazocin and consider one dose of aminoglycoside (if sensitive and not seriously unwell and after discussion with local microbiologist)
  • OPAT : Ertapenem
  • Daily IVOST review required
  • Potential IVOST options when afebrile, clinically improving, falling CRP and able to take and absorb oral medicines: ciprofloxacin, trimethoprim
  • There was less consensus on the role of pivmecillinam and its ability to penetrate renal tissue. It should be considered only to offer an oral option to finish a course of antibiotics in a clinically improving child

Scenario 3 – infection with ESBL producing Gram negative outside the renal tract (except for meningitis)

  • As per IDSA – carbapenem, meropenem (especially if child unwell)
  • Stepdown co-trimoxazole or ciprofloxacin (would consider fosfomycin on rare occasion)
  • Temocillin not empirical therapy until we understand PK/PD profile better plus expensive and difficult to obtain – consider not including
  • Recommendation would need caveat, how unwell, age of child.  Not experienced discuss with infection specialist. BNFC reference
  • Ertapenem OPAT

Scenario 4 – meningitis complicating ESBL producing Gram negative

  • Carbapenem, limited ability to step down
  • Treatment duration – 3 weeks
  • Lumbar puncture to ensure infection cleared.

[Conor Docherty]