by Christoph
Figure 1. Colony morphology of Granulicatella adiacens on 5% sheep blood agar 0.2 – 0.5 mm, white, non-hemolytic satellite colonies (lateral streak line) supported by colonies of Staphylococcus aureus (centre streakline). Frontpage: colonies of G. adiacens on 5% sheep blood agar (0.2 – 0.5 mm), satelliting around opaque, white, large colonies of Diphtheroids (=non-pathogenic Corynebacteria) after subculture from BacT/ Alert blood culture bottles. Source
Streptococcus pneumoniae and its ilk are among the first human pathogens discovered. Streptococci were first described by the Austrian surgeon Theodor Billroth in 1874, and Louis Pasteur showed in 1879 that they were the etiological agent (=microorganisms and microbial toxins that cause disease in humans) responsible for puerperal fever in women. Streptococcal infections vary in severity from mild throat infections to life-threatening infections of the blood (bacteremia ) or organs (pneumonia, meningitis, necrotizing fascitis ). Clinical microbiologists have often struggled but mostly found ways to cultivate streptococci in the lab (to reasonably cover this topic it would need an expert streptococcologist ). One report in 1961 described streptococci that could hardly grow even on special media but did reasonably well when grown in the proximity of other bugs (='satellitism' ), for example next to Staphylococcus aureus (Figure 1) or next to corynebacteria (see frontpage ). Apparently, they depend on metabolites secreted by their neighbors. What might these metabolites be? In the case of these streptococci, there is some evidence that they require pyridoxal (vitamin B6 ) and cysteine. These may in fact serve as early examples of what microbiologists are more and more aware of today: microbial life is a community enterprise. Originally, they were classified as 'nutritionally variant streptococci' (NVS), and later on you find them in the literature with the names of Granulicatella and Abiotrophia (Greek prefix ἄ (a)-, negative (un-); noun βιος (bios), life; noun τροφιά (trophia), nutrition), each genus having a handful named species. Species names and taxonomic affiliations have changed several times over the last years (I deliberately withhold the messy details ) until the issue was finally settled by taking into account their 16S rRNA phylogeny, which suggests that both represent different genera.
Figure 2. Site specificity of predominant bacterial species in the oral cavity. In general, bacterial species or phylotypes were selected on the basis of their detection in multiple subjects for a given site. Distributions of bacterial species in oral sites among subjects are indicated by the columns of boxes to the right of the tree as follows: not detected in any subject (clear box), <15% of the total number of clones assayed (yellow box), ≥15% of the total number of clones assayed (green box). The 15% cutoff for low and high abundance was chosen arbitrarily. Marker bar represents a 10% difference in nucleotide sequences (16S rRNA). Source
Although initially identified as pathogens, both Granulicatella and Abiotrophia are among the many bacteria that thrive on and in humans without doing any harm. They turn pathogenic only if they happen to get to the 'wrong places' (think of the commensal Escherichia coli that are part of the human gut microbiota but can turn really nasty when they get into the bloodstream by, for example, an injury). Granulicatella and Abiotrophia have been found as normal residents in the human oral cavity (Figure 2). Of the Gram-positives, Streptococcus mitis is found most often in all 'sample sites' of the oral cavity, closely followed by Gemella haemolysans and Granulicatella adiacens. Interestingly, these three have all been found to cause or be associated with endocarditis. If you wonder how these bacteria that quietly inhabit your mouth can cause heart inflammation ask your dentist: it is well known that periodontal abscesses can 'release' them into the bloodstream. Granulicatella and Abiotrophia have also been found associated with implanted prostheses, infections that are not easy to treat (as was recently experienced by Elio in one of his knees).
To give you another whiff of their 'family ties' among the Gram-positives: the DnaA proteins – responsible for initiation of chromosome replication – of Granulicatella and Abiotrophia are ~50% identical (BLASTp). Compare this to their identity with the DnaAs of Streptococcus mitis (43%), Gemella sanguinis (45%), Bacillus subtilis (46%), and that of the Gram-negative Gammaproteobacteria E. coli (40%). The DnaA proteins of E. coli K-12 and Pseudomonas aeruginosa PAO1, both of different families of the Gammaproteobacteria, share 63% identity. In another quick-and-dirty BLAST search, I saw much the same 'gradient' of protein sequence similarity for another key replication factor, DnaN, the beta-clamp of Dna Polymerase III. This suggests that Granulicatella and Abiotrophia are, at best, more distant cousins.
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