Infectious Disease – 4 (Below the Diaphragm)

We’ve already discussed causative Bugs & useful Drugs for infections of Skin & Soft Tissue, the Head & Neck, and the Lower Respiratory Tract.  Concluding now our topic of Infectious Disease, moving on south to abdomen & pelvis.

Infections below the diaphragm all potentially involve bacilli normal to the bowel.  They belong to the family Enterobacteriaceae, frequently referred to as “Enteric Organisms” (or just “Enterics”), or abbreviated GNR for “Gram-Negative Rods.”  We’ve previously addressed diarrheal pathogens (see our postings Acute Diarrhea and Chronic Diarrhea).  So now, let’s start with…


The following are the invariable GNR pathogens we find in UTIs:

Uncomplicated UTIs

  • Escherichia coli  [most common, by far]
  • Proteus mirabilis
  • Klebsiella pneumoniae

Only in Complicated UTIs

  • Serratia marcescens
  • Acinetobacter species
  • Enterobacter species
  • Pseudomonas aeruginosa

“Complicated” implies compromised host, indwelling catheter, neurogenic bladder, etc.  Of course, even in such patients, E. coli remains most common of all.

Staphylococci are invariably skin contaminants, except for Staph saprophyticus.  It’s implicated in from “hardly any,” to “up to 20% of” UTIs, depending on the study.  We won’t discuss it further, because all the antibiotics for GNRs will kill it as well.

I have a 65 y.o. male patient who’s grown “Coagulase-negative Staph, not saprophyticus” 3 times despite treatment.  He most likely has some source in the prostate gland, is awaiting urology consultation.  Consider any strange bug in a man’s C&S to be real.

Group B Streptococcus is often found in women — ignore it.  It’s normal vaginal flora, represents contamination.  In pregnant women, a urine C&S positive for Group B strep warrants antibiotic prophylaxis in labor, not for UTI’s, but as presumptive evidence of genital colonization which could cause fatal neonatal disease.

Antibiotics for enteric GNRs?  The following may theoretically all work:

  • Amp-/Amoxicillin  [but lots of resistance now]
  • Amoxacillin + clavulanate (Augmentin®)
  • Cephalosporins  [all generations]
  • TMP-SMX  [lots of resistance]
  • Doxycycline [tetracyclines]
  • Fluoroquinolones
  • Nitrofurantoin  (Macrobid®, Macrodantin®)
  • Fosfomycin  (Monurol®)
  • Gentamicin & other Aminoglycosides [in-pt. IV]
  • Carbapenems IV [for really severe infections]
  • Pipericillin, Carbenicillin [IV, for Pseudomonas]
  • Chloramphenicol [not used]

It may be easier to think about what doesn’t cover GNRs:

•  Penicillin                              •  Macrolides
•  Clindamycin                        •  Metronidazole

We treat most UTIs empirically, without ordering a C&S, because the infections are so common that we want to be cost effective.  But do obtain a C&S in the following circumstances:

•  Symptoms of Pyelo                             •  Men
•  Compromised Hosts                           •  Pregnant Women
•  Dysuria/urgency/frequency of >1 week duration
Why the last?  Because there’s evidence that within a week, untreated cystitis may well have ascended to the renal pelvis, causing subclinical Pyelo that may not respond to short-course therapy.

In terms of drugs, a few corollaries to consider:

**  Nitrofurantoin & Fosfomycin are heavily excreted into the bladder, but don’t penetrate renal parenchyma.  So NEVER use them if Pyelonephritis is a consideration (including patients with ≥7 days of symptoms).  Otherwise, these 2 drugs are excellent for cystitis, because they preserve other options for more worrisome patients, like those above.

**  Even if a bug is “resistant,” it may still work for cystitis.  That’s because MIC’s & Bauer-Kirby disks used in the C&S are calibrated to reflect serum drug levels.  Meds like TMP-SMX wind up heavily excreted in the urine, thus may well kill off whatever lurks in the bladder, but wouldn’t work in the renal parenchyma (where drug-delivery is via the bloodstream).

**  “ESBL” reported on a C&S means “Extended-Spectrum Beta-Lactamases.”  They’re reported as “pos” or “neg,” i.e. present or not.  ESBL’s will inactivate virtually all of our antibiotics, even if the report says “susceptible.”  Only Carbapenems such as Imipenem, Meropenem, Ertapenem, Doripenem, or Aztreonam (a monobactam) will work for sure, i.e. IV & expensive.  Actually, there are increasing (though still very few) reports of “CRE” (Carbapenem-Resistant Enterobacteriaceae).  Mortality is around 50%.

SO… which antibiotics should we use?  That mostly depends on resistance patterns in your area.  You can probably get some data from a local hospital, but that may well be nosocomial, or reflect sicker patients not generalizable to your practice.  Ideally, we’d all keep written records of how the C&S’s we order [usually for pyelo or pregnant women] turn out.

I pick Nitrofurantoin as my first choice when I treat cystitis empirically, because I know it can’t be used for pyelo.  If I think of Pyelo (see above), I order a C&S & pick empiric treatment to start.  I go with Cephalexin or Amoxicillin-clavulanate, simply because the majority of bugs in our area seem sensitive.  If there’s not much resistance where you practice, pick older drugs like Amoxicillin or TMP-SMX, so once resistance does develop, the other choices will be useful.  I rarely choose a quinolone; clinicians tend to give it out like water, so I try to save it.

Click for some common Antibiotic Doses (note that cystitis per se can be treated with 3-5 day courses).

That’s all for UTIs; hope I haven’t missed anything.  See also our postings Dysuria in Women and Dysuria in Men.  Onward to…


This means peritonitis — from a ruptured appy, other bowel perforations, or spontaneous bacterial peritonitis in ascitic fluid of cirrhotics.  These aren’t treated as out-patients, but one common entity is: diverticulitis.  The bugs are the same.

Like we noted, intra-abdominal pathogens are the enteric GNRs we find in the urinary tract, PLUS intestinal anaerobes which include Bacteroides fragilis.  As opposed to anaerobes above the diaphragm, ones here are universally resistant to penicillin.

Oral options for intestinal anaerobes:

•   Clindamycin                       •   Monifloxacin
•   Metronidazole                    •   Amoxicillin-Clavulanate

Unfortunately, Bacteroides is accumulating drug-resistance, such that only Metronidazole can truly be relied upon these days.  However, a recent study of uncomplicated diverticulitis found similar outcomes between antibiotics and placebo, suggesting the disease is self-limiting.  But the no-antibiotic subjects experienced more pain, so for now, treatment is still warranted.

To treat diverticulitis as an out-patient, we have to cover both enteric GNRs and anaerobes.  The following regimens can be used:

  • Ciprofloxacin + Metronidazole
  • Ciprofloxacin + Clindamycin
  • Amoxicillin-clavulanate [won’t cover Pseudomonas]
  • Monifloxacin

The latter is the only fluoroquinolone to cover anaerobes, though resistance may be emerging.


These include endometritis & salpingitis, lumped together as “Pelvic Inflammatory Disease” (PID).  And PID can be really complicated, depending on risk factors & antibiotic resistance factors.  As always, name the bugs:

  • STD Pathogens: Gonorrhea [GC], Chlamydia trachomatis (CT)
  • Intestinal Flora:  Enteric GNRs and Anaerobes
  • Vaginal Flora:  same as Intestinal, plus
    • Gardnerella, H. influenzae, Group B Strep

If you think a woman’s PID is due to an STD, then aim to cover everything, but don’t worry so much about Bacteroides resistance.  But if the PID is post-IUD insertion or other procedures, or post-partum, and there are no STD risks, be sure you’ve got the latter two categories covered well.

It seems like every year the recommendations for treating Gonorrhea change, due to increasing resistance.  Ceftriaxone 125 mg IM got bumped to 250 mg, & as of 2012 CDC recommends to add Azithromycin 1 gm as well [for GC alone, not for chlamydia per se].  Oral cefixime won’t fly any more.

For Chlamydia, it’s still Azithromycin 1 gm single-dose, or either Doxycycline 100 mg / Ofloxacin 300 mg (both BID x7 days).  Whew!!!  We saw above what we need for Intestinal organisms; fortunately the same drugs cover the extras in vaginal flora.

So our out-patient PID regimens might be:

Ceftriaxone 250 mg IM  +  Azithromycin 2 gm p.o. [GC & CT]
Cefoxitin 2 g IM   +  Probenecid 1 g p.o.  [GC]
Doxycycline 100 mg B.I.D. / Ofloxacin 300 BID (x14d)  [GNRs, CT]

These cover the STDs.  If the PID occurs after recent pelvic instrumentation, use Ofloxacin instead of Doxy (better GNR coverage) & add Metronidazole 500 mg B.I.D. x14d (anaerobes).

For the truly Pen-Allergic [angioedema, anaphylaxis], don’t risk a parenteral cephalosporin.  To cover GC, hospitalize for IV Clindamycin, then change to an oral regimen after improvement.

And that’s it for our discussions of Bugs & Drugs!  (See also our postings on infections of Skin & Soft Tissue, the Head and Neck, and the Lower Respiratory Tract).

Readers Please Note:  DiagnosisDude is out of town next week, then will be posting every 2 weeks as of 3/31/13.  Hopefully there’s enough existing content to keep readers busy.  Thanx so much to all regular followers over the last 54 weeks!!!

3 responses to “Infectious Disease – 4 (Below the Diaphragm)

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