Escaping bugs

Stop ceftriaxone and change to another antibiotic.

and

No.

Although the organism is sensitive to ceftriaxone it belongs to the ESCAPPM group of gram-negative bacteria:

• Enterobacter
• Serratia
• Citrobacter
• Actinetobacter
• Proteus
• Providentia
• Morganella

These bacteria like to throw a sucker punch by being sensitive to ceftriaxone on the initial culture and sensitivity report (they are all Enterobacteriaceae except Acinetobacter).  They all share a chromosomally-encoded cephalosporinase (a type of beta-lactamase) that is inducible. Thus they tend to develop cephalosporin resistance during treatment.

The cephalosporinase enzyme is not inhibited by beta-lactamase inhibitors like clavulinic acid.

Finally, for the uber-technical, the enzyme is an AmpC β-lactamase (Ambler molecular class C, Bush-Jacoby-Medeiros functional group 1). Other species may have similar resistance genes derived from this enzyme but carried on plasmids (e.g. E. coli and Klebsiella spp.).

In Australia the suggested initial antibiotic choice is:

gentamicin 4 to 6 mg/kg (child <10 years: 7.5 mg/kg; ≥10 years: 6 mg/kg) IV, daily (adjust dose for renal function)

and

amoxicillin 2g (child: 50 mg/kg up to 2 g) IV, 6-hourly

In the event of penicillin allergy, gentamicin alone is generally sufficient. In patients for whom gentamicin is inappropriate, third generation cephalosporins are the second-line empiric treatment:

ceftriaxone 1 g (child: 25 mg/kg up to 1 g) IV, daily

or

cefotaxime 1 g (child: 25 mg/kg up to 1 g) IV, q8h

but this does not provide adequate treatment for Enterococcus infections.

Antibiotics should continue for 10-14 days and be guided by culture and sensitivities (but remember the ESCAPPM caveat!). A follow up urine culture following the completion of treatment should be performed.