BUGS & DRUGS. Drugs aren’t exactly a part of “Diagnosis,” but Bugs certainly are. Matching the former to the latter is the essence of Infectious Disease. And I.D. represents our purest field of all, because it often results in cure. Aside from surgery, & perhaps some oncology, everything else within medicine simply alleviates suffering (e.g. treating pain, asthma, epilepsy, lupus, incontinence, etc.), or postpones it (managing diabetes, hypertension, etc.). Those lofty goals aren’t bad, by any means. But Antibiotics Cure!
So since I.D. can achieve so much, & infections may not be self-limiting like many other conditions, we want to get it right. Which means finding the most likely Diagnosis (Bug), in order to choose Drug.
We’ll discuss the most common organ systems we manage in out-patient care, barely (if at all) mentioning entities like meningitis or endocarditis which require hospitalization. Let’s start with…
SKIN & SOFT-TISSUE INFECTIONS (SSTI’s)
These infections include a variety of possibilities:
The Main Pathogens (The Bugs)
- Staphylococcus aureus (“Staph”)
- Streptococcus pyogenes (“Strep”)
We’ll use the abbreviations from now on. Click the link if you’d like a brief review of the Micro, including discussion of other species of staph & strep.
In out-patient care, Staph & Strep are our main targets. At the end, we’ll mention some special cases when we’d aim at other bugs. But in general, for any of the above SSTI’s, craft a regimen that will cover Staph &/or Strep.
In the 1940’s, Penicillin worked fine. But then Staph evolved to secrete an enzyme penicillinase, which destroys penicillin; fewer than 10% of strains are now sensitive. So new anti-Staph drugs were invented, which could defy penicillinase. The prototype was Methicillin (no longer available), others were 1st generation Cephalosporins. Another route was to simply inactivate penicillinase with Clavulanate (aka clavulanic acid), which was combined with simple Amoxacillin to then kill the bacteria.
Strep of course has always remained sensitive to plain-old penicillin. From ca. 1950 until just recently, the following antibiotics killed all Staph & Strep:1. Anti-Staphylococcal Penicillins (Methicillin class):
- Dicloxacillin (oral)
- Oxacillin (parenteral)
- Cephalexin (oral)
- Cefazolin (parenteral) [3rd-Gen. Ceftriaxone too]
3. Amoxicillin + clavulanate (Augmentin®)
But then a Staph strain developed resistance to Methicillin & all of the above, i.e. Methicillin-Resistant Staphylococcal Aureus, or MRSA [pronounced mér-sa] for short. Be careful reading the literature, where you’ll also see “MSSA”: “Methicillin-Sensitive Staph Aureus,” the non-resistant strain against which all the above antibiotics work (though clinicians never pronounce it as an abbreviation).
In San Francisco, MRSA has become so ubiquitous that I don’t bother ordering cultures any more (I probably should, to change & save anti-MRSA antibiotics). The only oral drugs that cover all Staph including MRSA are:
- Trimethoprim-Sulfamethoxazole (TMP-SMX) [Septra®, Bactrim®, et al.)
- Doxycycline (the main tetracycline available)
- Clindamycin (maybe; see below)
- Linezolid (at >$1,000)
Problem: TMP-SMX & Doxycycline don’t cover Strep. Clindamycin covers Strep just fine, but may miss some MRSA. If a Staph C&S reads “Sens” for Clinda, but “Resist” for Erythromycin, there’s a good chance the in vitro Clinda result is wrong. Order a “D-Test” to determine.
For patients ill enough to require hospitalization, there are plenty of parenteral options to cover these organisms, including Vancomycin & newer drugs like Teicoplanin, Daptomycin, Tigecycline, Quinupristin-dalfopristin, and Gentamicin. But as you suspect, these wily bacteria that have been around since pre-Cambrian days will surely adapt. Follow the literature for emerging reports of VISA (Vanco Intermediate Staph Aureus) & VRSA (you guessed it).
But for now, there’s mostly just MRSA to contend with. Since we may not be confidently able to give a single antibiotic that covers both Staph & Strep, we try to guess which bug it is. Clinical distinguishers include:
How to Treat (Out-Patient Setting)
In much of the country, MRSA may not be as prevalent as where I live. Traditional strategies may work just fine. Culture abscesses & other accessible infections to get an idea of resistance patterns in your area, if there’s no other data around for you. The Infectious Disease Society of America (IDSA) recently published its first Guidelines ever (click for link to free text).
MRSA not prevalent in community
- Dicloxacillin OR Amoxicillin-Clavulante
- Penicillin-Allergic: Cephalexin
- Initial parenteral dose: Cefazolin OR Ceftriaxone
The “initial parenteral dose” is when we hedge about admitting, think we can get by without, but want a really rapid treatment onset. See Table of Common Doses for all routes.
Can we give a cephalosporin to someone who’s penicillin-allergic? Older literature suggested 10% cross-reactivity; newer data place it closer to 2%. And that’s for the truly Pen-allergic. Probably 85% of people claiming a penicillin allergy do NOT have a true IgE-mediated condition. However, if the reaction included angioedema or anaphylaxis, I wouldn’t risk a cephalosporin [certainly not a parenteral one].
MRSA common in community
Make an educated diagnosis of Staph vs. Strep [see above]. If you think Strep, treat as you would if MRSA not an issue, because all the drugs cover Methicillin-sensitive Staph too (see above).
If you think Staph & thus deal with MRSA:
- Trimethoprim-Sulfamethoxazole (TMP-SMX)
- Clindamycin (if not a serious infection)
I used to use TMP-SMX all the time. But then I began to worry that over the years, I might cause a Stevens-Johnson or TENS. But if I use Doxy regularly, I may generate Pneumococcal resistance [Doxy plays a major role in treatment of pneumonia]. Use lots of Clinda, & I’ll see pseudomembranous colitis from C. difficile. So now I try to alternate among the three.
There’s no good “initial parenteral” option for MRSA. So I give Cefazolin or Ceftriaxone in such cases, plus two tablets of TMP-SMX DS from clinic stock.
I never use Linezolid because of a) cost; and b) last thing I want to see is “LRSA.” Save Linezolid for the hospitalized patient who never got a documented C&S result, & was sent home on it as oral Tx to finish the course.
1. NEVER give antibiotics for abscesses. Drain them. However, an abscess with large area of surrounding / extending cellulitis may need medical Tx too.
2. Failure to respond can be due to several possibilities:
- Wrong Diagnosis, e.g. rash is not an infection
- Abscess (needs I & D)
- Foreign body
- Underlying osteomyelitis
- Possibly – wrong Antibiotic
3. Some special infections involve other microorganisms:
||> Debilitated pts, esp. infected decubiti > Infected burns > Gangrene, fasciitis > Rose-bush infection (fungal) > Sea-water infection > Dog & Cat Bites > Cat-Scratch (esp. kittens) > Human bites|
Recurrent Staph Infections
Some patients keep getting Staph, often due to permanent nasal carriage or colonization from chronic skin diseases (psoriasis, eczema, etc.). It’s also important to rule-out two key diseases which predispose to recurrent SSTI’s:
Prevention depends on control of such underlying conditions. Otherwise, clinicians often try a couple of tricks:
- Topical Mupirocin (Bactroban®) in nose B.I.D. x5-10 days
- Chlorhexadrine (phisoHex®) total body washes for 5-14 days
- Both of above together
- Dilute household bleach baths (1 teaspoon per gallon of water (= ½ cup in ½ bathtub) for 15 min., 2x/wk for 3 mos. [be careful not to bathe for longer]
- Give a 10-14 day antibiotic treatment course, & add daily Rifampin 600 mg during the last 5 days (never give it stand-alone, or there’ll be immediate resistance)
Studies suggest the last doesn’t work. No good data on the others.
And that’s all for Skin & Soft-Tissue Infections; we may have at least penetrated the surface. Other I.D. postings address the variety of bacteria that infect the Head and Neck, the Lower Respiratory Tract, and organs Below the Diaphragm (UTIs, PID, & diverticulitis).