5.2 Bloodstream Pathogens and Skin Flora Significance

5.2 Bloodstream Pathogens and Skin Flora Significance

Quick Answer: Treat S. aureus, beta-hemolytic streptococci, S. pneumoniae, Enterobacterales, Pseudomonas, A. baumannii, and Candida in blood as pathogens. Judge CoNS, Cutibacterium, and Corynebacterium by number of sets positive, time to positivity, and clinical context. Separate viridans streptococci from S. pneumoniae with optochin and bile solubility, not by alpha hemolysis alone.

The M outline names a short list of organisms you must identify from blood and then asks a harder question: which of the skin flora that grow in the same bottles are contaminants, and which are endovascular pathogens? Colony morphology, a handful of rapid tests, and the epidemiology of the draw (how many sets, how fast, what hardware) answer both. This section is still outline II.A; it is the morphologic and interpretive half of the blood-culture bench.

Gram stain of the bottle sets the plate workup

A positive-bottle smear is already an identification method. Gram-positive cocci in clusters point to staphylococci (and occasionally Micrococcus from contamination). Cocci in chains or pairs point to streptococci and enterococci; lancet-shaped diplococci suggest S. pneumoniae. Gram-negative rods that are fat or bipolar may be Enterobacterales or Pseudomonas; short coccobacilli that resist decolorization and look Gram-variable are a classic Acinetobacter trap. Budding yeast, with or without pseudohyphae, is Candida until MALDI or a rapid yeast method says otherwise. Tiny Gram-negative coccobacilli send you back to the biosafety rules in the previous section, not onto an open-bench catalase rack.

Subculture choices follow the smear: sheep blood agar and chocolate for everything; MacConkey for Gram-negative rods; anaerobic blood agar if the anaerobic bottle flagged or the smear suggests clostridia or Cutibacterium. Do not wait for next-day colonies to call the critical Gram stain.

Staphylococcus aureus and other staphylococci

S. aureus colonies are medium to large, smooth, and creamy; many strains are golden-yellow and beta-hemolytic on sheep blood agar. Catalase is positive (do not pick from blood agar and call a weak bubble a real catalase—carryover peroxidase fools you). Tube or latex coagulase is positive. Mannitol salt agar turns yellow when the isolate ferments mannitol, a teaching reaction, not a blood-culture rapid method. In blood, S. aureus is a pathogen in a single bottle. Protein A and coagulase, covered with endocarditis in the next section, explain why it seeds valves and bones so efficiently.

Coagulase-negative staphylococci (CoNS)—S. epidermidis, S. hominis, S. haemolyticus, S. capitis, and others—make white or gray colonies that are usually nonhemolytic (S. haemolyticus can hemolyze). Catalase positive, coagulase negative. They are the most common blood-culture contaminants and, at the same time, the most common true pathogens of catheters and prosthetic valves. You cannot read clinical significance from the colony color.

Staphylococcus lugdunensis is the CoNS you must not shrug off. It is PYR positive and typically ornithine-decarboxylase positive, and it causes aggressive native-valve endocarditis and soft-tissue infection that behave like S. aureus. A “coagulase-negative staphylococcus” report without noticing lugdunensis is an exam (and patient) failure.

Micrococcus from skin is catalase positive, modified-oxidase (microdase) positive, and usually a contaminant; it is not on the outline’s pathogen list, but it is how a cluster-coccus contaminant looks when it is not a staphylococcus.

Beta-hemolytic streptococci, enterococci, and S. pneumoniae

Beta-hemolytic streptococci in blood are significant. S. pyogenes (group A) shows a wide zone of hemolysis, is PYR positive, and is bacitracin susceptible. S. agalactiae (group B) has a narrower zone, is CAMP and hippurate positive, and is a bloodstream and endocarditis pathogen in older adults as well as neonates. Group C/G (S. dysgalactiae) looks like group A on the plate and is also a real blood isolate. Catalase is negative; do not confuse hemolytic staphylococci with streptococci because you skipped catalase.

Enterococcus spp. form small gray colonies that are nonhemolytic or alpha-hemolytic (rarely beta). Catalase is negative or a weak pseudo-catalase. PYR is positive, growth in 6.5% NaCl is positive, and bile-esculin is positive (black). E. faecalis is still usually ampicillin susceptible; E. faecium is the vancomycin-resistant species you worry about with vanA. Enterococci in blood raise endocarditis and an abdominal or urinary source; they are not dismissed as skin flora.

Streptococcus pneumoniae is alpha-hemolytic, often mucoid from the capsule or “draughtsman/checker” from autolysis, and appears as lancet-shaped Gram-positive diplococci. Catalase is negative. It is optochin (P-disk) susceptible and bile soluble. A single blood isolate is always significant and should trigger a look at the lungs, meninges, and spleen. Viridans-group streptococci are also alpha-hemolytic but are optochin resistant and bile insoluble. That pair of tests—not colony wetness—is how the outline expects you to split them. Viridans streptococci in blood are endocarditis organisms, not “respiratory contaminants,” once you have more than a single late bottle in a patient without a dental-source syndrome.

Gram-negative rods and Candida

Enterobacterales are Gram-negative rods that are oxidase negative and ferment glucose. On MacConkey, E. coli is typically dry lactose-positive and indole positive; Klebsiella is mucoid lactose-positive and nonmotile; Proteus swarms and is H2S positive; Enterobacter and Serratia are hospital organisms with AmpC or other resistance stories. Any of them in blood is a pathogen. Endotoxin (LPS) from the outer membrane drives the septic-shock physiology that makes these bottles critical values even when the colony looks like everyday urine flora.

Pseudomonas spp., especially P. aeruginosa, prefer the aerobic bottle. Colonies spread, may look metallic, and classically smell like grapes or tortillas and make blue-green pyocyanin. Oxidase is positive; the organism is a nonfermenter and P. aeruginosa grows at 42°C. It is a catheter, burn, and neutropenia pathogen, not a skin contaminant.

Acinetobacter baumannii is a Gram-negative coccobacillus that can retain crystal violet and fool you into calling Gram-positive cocci on the bottle smear. Colonies are smooth, gray-white, and nonhemolytic; it grows on MacConkey as a non-lactose fermenter (sometimes slightly pink). Oxidase is negative, motility is negative, and it does not ferment glucose the way Enterobacterales do. It is a hospital, ventilator, and catheter organism and is often multidrug resistant. A blood isolate is significant.

Candida spp. appear as budding yeast in the Gram stain, sometimes with pseudohyphae (C. albicans, C. tropicalis) and sometimes as tiny yeasts without filaments (C. glabrata). Colonies on blood agar are creamy white, usually at 24–48 hours (glabrata can be slower). Germ-tube or PNA-FISH historically separated C. albicans; MALDI now speciates the common yeasts, including C. auris, which is an infection-control emergency. Yeast in a blood-culture bottle is not skin contamination. Malassezia is the rare lipid-requiring exception associated with lipid infusions; it is not an excuse to sit on a creamy white yeast from a routine bottle.

Skin flora: true pathogen versus contaminant

The outline’s “species comprising skin flora” are the organisms that live in follicles and on the stratum corneum and therefore colonize imperfectly prepped venipuncture sites and catheter hubs:

• CoNS, especially S. epidermidis
• Cutibacterium (formerly Propionibacterium) acnes* — anaerobic Gram-positive rod, catalase positive, indole positive, often flags late (day 3–5) from shoulder or valve hardware
• Corynebacterium spp. — irregular Gram-positive rods in Chinese-letter or palisade arrangements, catalase positive, usually nonhemolytic gray-white colonies

These organisms can cause true bloodstream and endovascular infection, particularly with prosthetic valves, pacemaker leads, central lines, and neutropenia. They are not automatically pathogens. Use three questions together, never one:

1. How many sets are positive? One bottle of four with CoNS is usually a contaminant. Two or more separate sets with the same organism support infection.
2. How fast did the bottle flag? True pathogens often flag within 24 hours. C. acnes and some corynebacteria that are contaminants often flag near the end of the 5-day protocol. A 10-hour CoNS flag from a new prosthetic valve is a different conversation from a 96-hour flag in a patient who is already well.
3. What is the clinical context? Fever, chills, a tender tunnel, a prosthetic valve, or no other source pushes toward pathogen. A single late CoNS in a patient cultured “because we always draw cultures” pushes toward contaminant.

Do not apply this three-question test to S. aureus, S. pneumoniae, beta-hemolytic streptococci, Enterobacterales, Pseudomonas, A. baumannii, or Candida. Those organisms do not get a free contaminant pass from a single bottle.

The blood-culture bench is therefore two jobs at once: name the colony with catalase, coagulase, optochin, bile, oxidase, and yeast morphology, then decide whether the name you just issued is a critical pathogen or a carefully worded possible contaminant. Both jobs are on the M exam.