As mentioned last week, I’m currently attending on the general medical service, a chance to brush up on non-ID clinical skills, and more importantly, to work with smart, energetic house staff and medical students.
Not surprisingly, there’s a wide range of clinical ID on this service, and this year we’ve had a rash of streptococcal infections.
(Get it — “rash” and “strep”? Ha ha ha, medical puns are so funny.)
These cases motivated the following summary of clinically important streptococcal infections. They’re worth reviewing because they are common and can be severe; additionally, streptococcal taxonomy has got to be one of the most complicated, confusing, and ever-changing areas in all of microbiology.
It seems to have always been this way with strep — I remember struggling with the right nomenclature back during ID fellowship, and it hasn’t become easier over time. Some of these bacteria have had many names over the years, all synonymous and used interchangeably in medical conversations and literature without rhyme or reason.
Sorry about that. Maybe we should send a letter of protest to the International Association of Streptococcal Taxonomic Standards, which doesn’t really exist — but if it did, they’d deserve our cranky petition.
Here then, is a “Strep Infections 101” course — the ones you should remember, and why, with perhaps a few fun taxonomic facts thrown in. Fun, at least, to ID geeks like me.
And since I’m neither a microbiologist nor a super-specialist in bacterial infections, apologies ahead of time for any errors.
- Invasive beta-hemolytic streptococci: Named “beta strep” by their characteristic clearing when they grow on blood agar (for some ancient reason called “beta hemolysis”), they are further classified by their Lancefield antigen status, referring to their carbohydrate composition of their cell walls. These include most importantly Strep pyogenes (group A strep, GAS) and Strep agalactiae (group B strep, GBS). The former is the well-known cause of pharyngitis and skin infections — everything from cellulitis to erysipelas to lymphangitis to necrotizing fasciitis (it’s the dreaded “flesh-eating bacteria”). Strep pyogenes can also cause scarlet fever (see rash “joke” above), and is unique among these bacteria in causing rheumatic fever. Strep agalactiae is best known as a pathogen of newborns (and for being difficult to spell) — but with our strategy of giving prophylaxis to pregnant mothers who are colonized with GBS, it’s actually more common now as a cause of invasive disease (bacteremia, sepsis, osteomyelitis) in adults, most of whom have comorbid medical conditions such as diabetes, alcoholism, or liver disease. Three other lettered beta-streps (groups C, F, and G) cause pharyngitis and clinically important invasive infections, but we hardly ever refer to them by their species names — weirdly, just their letter (for example, “beta-hemolytic streptococcus group G”). Therapeutically, a welcome characteristic of all these beta streps is that they remain susceptible to penicillin and most cephalosporins — hooray!
- Strep pneumoniae. Once upon a time this would have been listed #1! However, though still one of the leading causes of otitis media, sinusitis, pneumonia, and meningitis, the incidence has dramatically declined due to our two available pneumococcal vaccines. Particularly susceptible hosts include those with defects in humoral immunity or asplenia — myeloma, HIV, certain congenital immunodeficiencies, sickle cell disease, alcoholism. Penicillin susceptibility has gradually declined (though most are still penicillin susceptible), while respiratory fluoroquinolone activity fortunately (and surprisingly) has not, at least here in the USA.
- Endocarditis-causing “Strep viridans”: Normal inhabitants of the mouth, and known primarily as a common cause of endocarditis, the viridans streptococci are actually multiple different species of strep frequently lumped under the pseudotaxonomic name, Strep viridans. Many have alpha (green) hemolysis on blood agar, from which they derive their name (viridis is Latin for green); some have no hemolysis at all. Microbiology purists don’t generally like pseudotaxonomic names, and hence your lab will report something more specific than Strep viridans, e.g. Strep mutans, Strep salivarius, Strep mitis, etc.
- Also endocarditis-causing Strep bovis — I mean Strep gallolyticus: I nearly put Strep bovis in the above group since it has also become a pseudotaxonomic name; turns out these bacteria previously identified as Strep bovis are actually multiple different species — Strep gallolyticus, Strep infantarius, probably others. Since I don’t really understand all this, and only have so much time in the day, I was heartened to read the following in a recent review: “The history of the Streptococcus bovis group is complicated and confusing due to conflicting classical distinctions based on imperfectly differentiating phenotypic attributes and due to modern disagreements concerning the optimal molecular methods for identification to the species level.” That’s for sure! Endocarditis with Strep bovis (ok, Strep gallolyticus) is most famously associated with colon cancer — a fact that almost 100% of even apathetic medical students know. The penetrance of this bit of microbiology trivia might even surpass medical students’ knowledge of listeria. Which makes me wonder — why are some random facts so memorable? And will this sticky Strep bovis knowledge persist with the new name?
- Abscess-forming streptococci — Strep anginosus, Strep intermedius, and Strep constellatus: Formerly known simply as Strep milleri (those were the days), these streptococci also turn out to be multiple different species, and are now named after one of the three principal isolates, Strep anginosus — hence the term Strep anginosus group is preferred over Strep milleri. In addition to Strep anginosus, you might also find Strep intermedius or Strep constellatus — all three are common causes of dental, lung, liver, brain, and other abscesses. (These species also may have subspecies, but I’m not going to go there. You’re welcome.) As with the beta streps, they have retained penicillin and cephalosporin susceptibility — ceftriaxone is the drug we most commonly use for treatment.
There are, of course, numerous other streptococcal infections, but these are the most common ones on a general medical service.
Plus, it’s time to watch a big inflatable duck rolling down a highway in Des Moines.