A 62‑year‑old man with poorly‑controlled type 2 diabetes (HbA1c 9.4 %) is admitted with a worsening plantar ulcer under the right first metatarsal head.
Two weeks ago his podiatrist prescribed trimethoprim–sulfamethoxazole (TMP‑SMX) for presumed mild cellulitis. The ulcer has enlarged, now draining malodorous pus.
Vitals: T 38.3 °C, HR 96 bpm, BP 132/78 mm Hg.
Exam: 3 × 2 cm ulcer, erythema extending 4 cm, no bone palpated. Peripheral pulses intact.
Labs: WBC 13 000/µL (85 % neutrophils), CRP 110 mg/L, Cr 1.0 mg/dL.
X‑ray: soft‑tissue swelling; no osteitis.
You plan empiric IV therapy while awaiting deep‑tissue culture.
Which regimen is most appropriate now?
A. Continue TMP‑SMX and add oral clindamycin
B. Vancomycin plus cefepime
C. Piperacillin–tazobactam (Zosyn)
D. Linezolid monotherapy
E. Ciprofloxacin plus metronidazole
Correct Answer — C. Piperacillin–tazobactam
Why this is the best choice
Reliable Streptococcus coverage (including group B) → addresses the key failure point of TMP‑SMX.
Excellent MSSA cover and decent Gram‑negative activity (Enterobacterales and Pseudomonas), handy in diabetic foot where polymicrobial infection is common.
Anaerobes covered (esp. in foul‑smelling or deep wounds).
No MRSA risk factors are evident → vancomycin is unnecessary at presentation, sparing nephrotoxicity and monitoring.
IDSA 2023 diabetic‑foot guidelines list piperacillin‑tazobactam as a first‑line empiric agent for moderate–severe infections when Pseudomonas or resistant Gram‑negatives are plausible.
Option‑by‑Option Critique
| Option | Why it’s incorrect / sub‑optimal |
|---|---|
| A. TMP‑SMX + clindamycin | • TMP‑SMX lacks predictable activity against β‑haemolytic streptococci (A, B, C, G)—the likely culprit in this failure. Clindamycin adds anaerobe and strep cover, but the combination still leaves gaps against Gram‑negative rods and risks inducible clindamycin resistance in staph. • Overall, spectrum is patchy and still oral in a patient who now meets criteria for IV therapy. |
| B. Vancomycin + cefepime | • Broad cover for MRSA + Gram‑negatives (incl. Pseudomonas), but vancomycin adds unnecessary nephrotoxicity when the patient has no MRSA risk (no prior MRSA, health‑care exposure, or severe infection pattern). • Lacks anaerobic cover → would still need metronidazole or clindamycin. • “Over‑treats” and complicates stewardship. |
| C. Piperacillin‑tazobactam | Chosen answer – broad one‑stop regimen that reliably treats streptococci, Gram‑negatives (inc. Pseudomonas) and anaerobes, matching real‑world resistance patterns in diabetic foot infection. |
| D. Linezolid | • Purely Gram‑positive; no Gram‑negative or anaerobic cover. • Excellent for MRSA/enterococcus but unnecessary here and expensive with haematological toxicity risk. |
| E. Ciprofloxacin + metronidazole | • Ciprofloxacin has poor streptococcal activity (same Achilles’ heel as TMP‑SMX) and increasingly unpredictable Gram‑negative sensitivity in diabetics. • Metronidazole adds anaerobes only; streptococci remain untreated. |
Take‑home Messages
Streptococcus agalactiae (GBS) is a frequent pathogen in diabetic foot infections and is not reliably covered by TMP‑SMX or ciprofloxacin.
Choose an empiric regimen that covers strep, MSSA ± MRSA (per risk), Gram‑negative rods, and anaerobes.
Piperacillin‑tazobactam offers a pragmatic single‑agent solution when resistance to ampicillin‑sulbactam among Enterobacterales or Pseudomonas is a concern.
Escalate or de‑escalate once culture results and clinical response are available to optimise stewardship.
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