S. pyogenes' skin infection diagnostics

S. pyogenes is "flesh-eating" bacteria. It results from life-theatening myonecrosis caused by this organism. S. pyogenes avoids phagocytosis (mediated primarily by capsule, M and M-like proteins, C5a peptidase), adhere to and invade host cells (M protein, lipoteichoic, F protein), and produce toxins (streptococcal pyrogenic exotoxins, steptolysin S, streptolsin O, streptokinase, DNases).

Suppurative diseases: pharyngitis, soft-tissue infections.

Erysipelas: localised skin infection with pain, inflammation, lymph node enlargement and systemic symptoms.

Cellulitis: infection of the skin that involves the subcutaneous tissues.

Necrotising fasciitis: deep infection of skin that involves destruction of muscle and fat layers.

I think these are most its skin infections. But what about its diagnostics?

I think take biopsy from living tissue, not from necrotic. If pus, then probably from it. I think we can do Gram staining, recognise gram-positive cocci like in a string Then, we can do hydrogen peroxide test - if no bubbles, then suspicion to S. pyogenes, since catalase negative. The do optochin test which is positive for S. pyogenes.

What is the correct procedure for doing skin infection diagnostics for group A streptococci?

S. pyogenes cultured on Blood Agar should show beta-hemolysis. B-hemolytic colonies can be further identified as S. pyogenes by the results of negative catalase (hydrogen peroxide) test, positive L-pyrrolidonyl arylamidase (PYR) reaction (1) and sensitivity to Bacitracin (2). See

PCR may also be used:

Streptococcus pyogenes

Streptococcus pyogenes , also known as group A streptococcus (GAS), is most commonly associated with mild, self-resolving infections of the skin and oropharynx. However, dissemination of the bacteria to normally sterile sites within the body can lead to a variety of invasive conditions that are associated with high morbidity and mortality. In addition, the generation of human cross-reactive antibodies in response to lingering GAS infection can result in the development of post-streptococcal autoimmune sequelae that afflict the organs, joints and CNS.

GAS pathogenesis is mediated by an extensive repertoire of extracellular virulence factors. Initial colonization of the skin and oropharynx is facilitated by cell-associated adhesins that bind to multiple components of the host extracellular matrix. While a battery of antiphagocytic molecules allow the organism to persist at the initial site of infection, the production of multiple toxigenic and tissue-destructive virulence factors facilitates the transition from a superficial to an invasive disease phenotype.

Despite the continuing susceptibility of GAS to β-lactam antibiotics a resurgence of serious streptococcal disease has been observed over the past 30 years. While the cause of this resurgence is incompletely understood it has been tentatively attributed to the reappearance and/or increased circulation of a highly invasive clone of serotype M1T1 GAS. This shift in the epidemiology of GAS infection highlights the need for increased surveillance of GAS in the community, faster, more reliable diagnostic tests for GAS infection in a clinical setting, and more targeted treatments of invasive GAS disease. Above all, the development of a safe, effective GAS vaccine would prove invaluable.


Cystitis is most often caused by a bacterial infection of the bladder, but it can also occur as a reaction to certain treatments or irritants such as radiation treatment, hygiene sprays, or spermicides. Common symptoms of cystitis include dysuria (urination accompanied by burning, discomfort, or pain), pyuria (pus in the urine), hematuria (blood in the urine), and bladder pain.

In women, bladder infections are more common because the urethra is short and located in close proximity to the anus, which can result in infections of the urinary tract by fecal bacteria. Bladder infections are also more common in the elderly because the bladder may not empty fully, causing urine to pool the elderly may also have weaker immune systems that make them more vulnerable to infection. Conditions such as prostatitis in men or kidney stones in both men and women can impact proper drainage of urine and increase risk of bladder infections. Catheterization can also increase the risk of bladder infection (see Case in Point: Cystitis in the Elderly).

Gram-negative bacteria such as Escherichia coli (most commonly), Proteus vulgaris, Pseudomonas aeruginosa, and Klebsiella pneumoniae cause most bladder infections. Gram-positive pathogens associated with cystitis include the coagulase-negative Staphylococcus saprophyticus, Enterococcus faecalis, and Streptococcus agalactiae. Routine manual urinalysis using a urine dipstick or test strip can be used for rapid screening of infection. These test strips (Figure (PageIndex<1>)) are either held in a urine stream or dipped in a sample of urine to test for the presence of nitrites, leukocyte esterase, protein, or blood that can indicate an active bacterial infection. The presence of nitrite may indicate the presence of E. coli or K. pneumonia these bacteria produce nitrate reductase, which converts nitrate to nitrite. The leukocyte esterase (LE) test detects the presence of neutrophils as an indication of active infection.

Low specificity, sensitivity, or both, associated with these rapid screening tests require that care be taken in interpretation of results and in their use in diagnosis of urinary tract infections. Therefore, positive LE or nitrite results are followed by a urine culture to confirm a bladder infection. Urine culture is generally accomplished using blood agar and MacConkey agar, and it is important to culture a clean catch of urine to minimize contamination with normal microbiota of the penis and vagina. A clean catch of urine is accomplished by first washing the labia and urethral opening of female patients or the penis of male patients. The patient then releases a small amount of urine into the toilet bowl before stopping the flow of urine. Finally, the patient resumes urination, this time filling the container used to collect the specimen.

Bacterial cystitis is commonly treated with fluoroquinolones, nitrofurantoin, cephalosporins, or a combination of trimethoprim and sulfamethoxazole. Pain medications may provide relief for patients with dysuria. Treatment is more difficult in elderly patients, who experience a higher rate of complications such as sepsis and kidney infections.

Figure (PageIndex<1>): A urine dipstick is compared against a color key to determine levels of various chemicals, proteins, or cells in the urine. Abnormal levels may indicate an infection. (credit: modification of work by Suzanne Wakim)


Robert, an 81-year-old widower with early onset Alzheimer&rsquos, was recently moved to a nursing home because he was having difficulty living on his own. Within a few weeks of his arrival, he developed a fever and began to experience pain associated with urination. He also began having episodes of confusion and delirium. The doctor assigned to examine Robert read his file and noticed that Robert was treated for prostatitis several years earlier. When he asked Robert how often he had been urinating, Robert explained that he had been trying not to drink too much so that he didn&rsquot have to walk to the restroom.

All of this evidence suggests that Robert likely has a urinary tract infection. Robert&rsquos age means that his immune system has probably begun to weaken, and his previous prostate condition may be making it difficult for him to empty his bladder. In addition, Robert&rsquos avoidance of fluids has led to dehydration and infrequent urination, which may have allowed an infection to establish itself in his urinary tract. The fever and dysuria are common signs of a UTI in patients of all ages, and UTIs in elderly patients are often accompanied by a notable decline in mental function.

Physical challenges often discourage elderly individuals from urinating as frequently as they would otherwise. In addition, neurological conditions that disproportionately affect the elderly (e.g., Alzheimer&rsquos and Parkinson&rsquos disease) may also reduce their ability to empty their bladders. Robert&rsquos doctor noted that he was having difficulty navigating his new home and recommended that he be given more assistance and that his fluid intake be monitored. The doctor also took a urine sample and ordered a laboratory culture to confirm the identity of the causative agent.

  1. Why is it important to identify the causative agent in a UTI?
  2. Should the doctor prescribe a broad-spectrum or narrow-spectrum antibiotic to treat Robert&rsquos UTI? Why?

Disruption of the cutaneous barrier, such as presence of ulcers, wounds, or fungal skin infections (e.g., athlete&rsquos foot), is a risk factor for developing cellulitis. 1,4,5 Previous history of cellulitis venous insufficiency, presence of chronic edema, or impaired lymphatic drainage of the limbs obesity and injection drug use have also been identified as risk factors for cellulitis. 1,4,6

Diagnosis of cellulitis is usually made clinically.

For cellulitis, the Infectious Diseases Society of America (IDSA) does not recommend routine collection of cultures, including blood, cutaneous aspirates, biopsies, or swabs. 7 However, blood culture and microbiologic examination and culture of cutaneous aspirates, biopsies, and swabs may help when atypical pathogens are suspected. These procedures are recommended by IDSA in those with immunocompromised status, immersion injuries, or animal bites. 7 Waiting for culture results should never delay the initiation of treatment however, when available, culture results can be used to tailor antibiotic therapy. Streptococcus pyogenes

Note gram-positive (purple) cocci in chains (arrows).

Figure (PageIndex<1>): Strep. pyogenes is beta-hemolytic. It can completely break down blood in blood agar plates, leaving just the color of the base medium (similar to nutrient agar or plate count agar). Hemolysis is a common method of distinguishing among groups of streptococci. (Rebecca Buxton. 2005. ery/image.2881)

  • Pharyngitis is pread person to person primarily by respiratory droplets skin infections are spread by direct contact with an infected person or through fomites (contaminated inanimate objects).


  • The group A beta hemolytic streptococci are responsible for most acute human streptococcal infections. Between 5% and 20% of children are asymptomatic carriers. The most common infection is pharyngitis with the organism usually being limited to the mucous membranes and lymphatic tissue of the upper respiratory tract. Children are at greatest risk for infection.
  • The most common infection is pharyngitis (strep throat) with the organism usually being limited to the mucous membranes and lymphatic tissue of the upper respiratory tract. Characteristic pockets of pus typically form on the tonsils (pyogenes means "pus-making")
  • From the pharynx, however, the streptococci sometimes spread to other areas of the respiratory tract resulting in laryngitis, bronchitis, pneumonia, and otitis media.
  • Scarlet fever:
    • accompanies streptococcal pharyngitis
    • diffuse red rash beginning on chest and spreading to the rest of the body after rash disappears, skin sloughs off
    • tongue becomes bright red
    • caused by pyrogenic toxins of some strains
    • Certain strains of S. pyogenes cause invasive group A beta streptococcal infections. Each year in the U.S. there are between 750 and 1500 cases of necrotizing fasciitis where a streptococcal-coded protease called Exotoxin B destroys the muscle (myositis) or the muscle covering (necrotizing fasciitis).
    • S. pyogenes are introduced into compromised skin (cut, scrape, or other wound)
    • Commonly called "flesh-eating disease"
    • Initial symptoms include redness, swelling, and intense pain at the site of infection
    • Later symptoms include distended and discolored skin, fever, nausea, vomiting, low blood pressure, and mental confusion
    • Can progress extremely rapidly
    • Affected tissue must be removed completely and the patient treated with broad-spectrum antibiotics
    • Subsequent to the initial infection
    • Thought to be autoimmune diseases caused by antibodies made against streptococcal antigens cross react with human tissues
    • Rheumatic fever
      • Antibodies react with joint membranes and heart valves
      • Common symptoms are fever, joint pain, heart murmur, fatigue, and small, painless bumps under the skin
      • Most common in 5-15 year olds
      • Results in life-long heart valve damage
      • Anitbodies react against glomerular cells and basement membranes of the kidneys
      • Symptoms include high blood pressure, low urine output, and blood and proteins in urine
      • Children usually completely recover adults may have permanent damage

      Primary Virulence Factors

      • Similar to Staph. aureus, Strep. pyogenes has a variety of virulence factors, some of which (such as the pyrogenic (meaning "fever generating) exotoxins) are only found in certain strains (Figure (PageIndex<2>))

      Figure (PageIndex<2>): Streptococcus pyogenes virulence factors. (2021 Jeanne Kagle)

      From Streptococcus Group A Infections, by Sat Sharma, MD, FRCPC, FACP, FCCP, DABSM, Program Director, Associate Professor, Department of Internal Medicine, Divisions of Pulmonary and Critical Care Medicine, University of Manitoba Site Coordinator of Respiratory Medicine, St Boniface General Hospital and Godfrey Harding, MD, FRCPC, Program Director of Medical Microbiology, Professor, Department of Medicine, Section of Infectious Diseases and Microbiology, St Boniface Hospital, University of Manitoba, Canada.

      Regulatory Aspects of GAS Biofilms

      The biofilm lifestyle is associated with broad transcriptional changes, affecting the expression levels of about 25% of the GAS genes (Cho and Caparon, 2005). Several transcriptional regulators were shown to be involved in and crucial for the establishment and maintenance of biofilms. From the data available to date, three major regulatory processes can be deduced that facilitate the biofilm lifestyle of GAS:

      (i) Peptide pheromone based quorum sensing mediated by the short hydrophobic peptides SHP2/SHP3 (Chang et al., 2011).

      (ii) Repression of secreted and surface associated enzymes such as the cysteine protease SpeB and other proteases and nucleases (Dmitriev et al., 2008 Roberts et al., 2010a Connolly et al., 2011a McDowell et al., 2012).

      (iii) Induction of surface associated autoaggregative and adhesive structures such as M- and M-like proteins and the FCT region encoded pilus (Cho and Caparon, 2005 Luo et al., 2008 Manetti et al., 2010).

      The major players and the regulatory network contributing to GAS biofilm formation are summarized in Figure 2.

      Figure 2. Regulatory network involved in GAS biofilm formation. Arrow heads indicate direct or indirect induction, blocked lines indicate direct or indirect repression, dashed lines indicate export out of the bacterial cell, and dotted lines indicate ambiguous effects. Outer circle (light blue): transcriptional regulation level Inner circle (darker blue): biofilm-associated virulence factors Outside: environmental conditions and quorum sensing peptides influencing the biofilm phenotype. “?” stands for unknown Regulator/regulatory mechanism.

      Quorum Sensing

      Quorum sensing mechanisms are crucial for biofilm formation in many organisms. In GAS, four different ways of inter- and intraspecies communication are described, i.e., Rgg-, Sil-, lantibiotics-, and LuxS/Autoinduer-2-dependent processes (Jimenez and Federle, 2014).

      In GAS, biofilm formation is associated with peptide-pheromone based quorum sensing mediated by the short hydrophobic peptide (SHP) pheromones SHP2 and SHP3. These peptide pheromones are encoded downstream of two genes encoding for the Rgg-like transcriptional regulators Rgg2 and Rgg3, respectively (Chang et al., 2011 Federle, 2012 Lasarre et al., 2013 Aggarwal et al., 2014). The propeptides are secreted and processed to the mature peptide pheromones SHP2C8 and SHP3C8, which are taken up into GAS via the oligopeptide permease Opp. The transcription of both peptide pheromone genes shp2 and shp3 is inhibited as long as Rgg3 is bound to the respective promoters. SHP2C8 and SHP3C8 bind to Rgg3 and Rgg2, leading to a dissociation of Rgg3 from and binding of Rgg2 to the shp2 and shp3 promoters. In a positive feedback loop, this induces the expression of shp2 and shp3 (Chang et al., 2011 Aggarwal et al., 2014). In GAS M49 NZ131 it has been shown that SHP2/3 dependent activation via Rgg2 induces biofilm production, while Rgg3 represses biofilms via repression of SHP2/3 production. It is not known to date, which transcriptional changes are caused by the SHP2/3 dependent activation of Rgg2 and inactivation of Rgg3 that finally lead to biofilm formation. Furthermore, it has not been elucidated yet whether this system also controls biofilm formation in other GAS strains, but in silico analyses show that Rgg2 and Rgg3 are present in all GAS strains (Chang et al., 2011).

      Two of the other above-mentioned quorum sensing systems of GAS have been associated with the GAS biofilm lifestyle as well. For an M18 strain it could be shown that a SilC deletion mutant was significantly impaired in biofilm formation (Lembke et al., 2006). Furthermore, there are hints that LuxS is involved in the control of SpeB production and emm gene expression, which could influence biofilm formation (Lyon et al., 2001 Marouni and Sela, 2003 Siller et al., 2008 Beema Shafreen et al., 2014). Both of the latter QS systems have not been investigated in the context of GAS biofilm in detail yet. For more details on GAS quorum sensing please refer to a current review by Jimenez and Federle (Jimenez and Federle, 2014).

      Transcriptional Regulators of SpeB and Other Secreted Enzymes

      Since SpeB activity leads to dispersal of biofilm structures and prevents biofilm formation in GAS, repression of speB transcription is necessary for successful biofilm establishment (Doern et al., 2009). Therefore, regulators involved in transcription of speB also control biofilm formation in GAS. Transcriptional regulation of SpeB is quite complex and involves direct and indirect actions of numerous GAS regulators, as recently reviewed by Carroll and Musser (2011). Positive regulators directly acting at the promoter of the speB gene are RopB, another member of the Rgg-regulator family also referred to as Rgg1 (Chaussee et al., 1999 Neely et al., 2003 Dmitriev et al., 2008 Hollands et al., 2008), and the sugar metabolism regulator CcpA (Kietzman and Caparon, 2010 Shelburne et al., 2010). Consequently, deletion of the ropB gene leads to lower speB expression and an increased biofilm formation as shown in the M49 NZ131 strain (Chang et al., 2011). To our knowledge, for CcpA an influence on biofilm formation has not been elucidated yet.

      The CovR (aka CsrR) response regulator of the CovRS two component system probably binds directly to the speB promoter as well, acting as a transcriptional repressor (Miller et al., 2001). Consequently, repression of speB transcription by CovR enables GAS biofilm formation. CovRS influence on biofilm formation seems to be serotype or even strain dependent. It has been shown that deletion of the sensor kinase CovS leads to decreased biofilm formation in most strains tested. However, for some M6 strains an increased biofilm formation has been observed in CovS deletion strains (Hollands et al., 2010 Sugareva et al., 2010). Furthermore, it was shown that a mutant of the HSC5 strain lacking the CovR response regulator is unable to form biofilm at all (Cho and Caparon, 2005).

      Another virulence-associated regulator, Srv, is involved in control of of speB expression via indirect mechanisms (Reid et al., 2004 Doern et al., 2009 Roberts et al., 2010a Connolly et al., 2011a). The deletion of srv in the M1T1 strain MGAS5005 leads to an increased activity of SpeB and therefore to loss of the biofilm phenotype (Reid et al., 2006 Doern et al., 2009). In Western Blot analyses SpeB could not be detected in MGAS5005 biofilms after 24 h growth, whereas in the srv deletion mutant high amounts of SpeB are present in cultures after 24 h growth (Doern et al., 2009). The Srv mediated repression of SpeB activity is not restricted to the MGAS5005 strain, which has a naturally occuring mutation that leads to an inactive CovS sensor kinase. The effects of Srv on SpeB and biofilm production have also been observed for other GAS strains, although effects of srv deletion are not as drastic in those strains as they are in MGAS5005 (Connolly et al., 2011a).

      Another regulator potentially involved in biofilm formation is CodY, a regulator involved in the response to nutrient deprivation in many gram positive bacteria (Sonenshein, 2005). CodY deletion mutants were shown to have a reduced biofilm formation capacity of GAS in chemically defined medium (McDowell et al., 2012). This effect probably also results from the indirect CodY-mediated repression of the production of SpeB and other secreted proteases and nucleases (McDowell et al., 2012).

      Transcriptional Regulation of Biofilm-Relevant MSCRAMMS

      The transcriptional regulation of GAS surface associated adhesins has been subject to extensive investigations and the regulatory networks have often been reviewed in the past (Kreikemeyer et al., 2003 Hondorp and McIver, 2007 McIver, 2009 Fiedler et al., 2010). Nevertheless, only few of the regulators involved have been investigated with respect to their impact on biofilm formation. Since biofilm formation is apparently associated with the pilus and the M-protein family, it is quite obvious that transcriptional regulators influencing the expression of the FCT region encoded pilus genes and the emm gene should influence biofilm formation in GAS. Mga is the major stand-alone transcriptional positive regulator of emm and emm-like genes (Hondorp and McIver, 2007). Consequently, Mga inactivation leads to a loss of autoaggregation and biofilm formation capacity in GAS (Cho and Caparon, 2005 Luo et al., 2008). Regulation of Mga itself is very complex and was recently reviewed (Hondorp and McIver, 2007 Patenge et al., 2013).

      For some strains, i.e., those harboring an FCT-2, -3, or -4 type pilus encoding region, one of the major environmental signals driving biofilm formation is the external pH, as shown exemplarily for an FCT type 3 strain in Figure 3. In these strains, pilus expression is induced under acidic conditions. In contrast, FCT-1 strains produce pH-independent biofilms and do not show any pH-dependent differences in pilus gene expression (Köller et al., 2010 Manetti et al., 2010). The regulator(s) mediating the pH-driven expression of the pilus genes are not known yet. It is likely that the FCT-region encoded RofA-like regulators RofA or Nra might be involved, although this has not been experimentally proven yet (Kreikemeyer et al., 2002, 2011).

      Figure 3. Confocal Laser Scanning micrographs of 24 h emm3/FCT-3 GAS strain HRO-K-044 biofilms cultured in alkalined or acidified C-medium. Cells were stained with live/dead dye containing Syto9 and Propidiumiodide. Magnification 630 times box size: 19.8 × 19.8 μm. Left panel: mature biofilm grown in C-medium with initial pH of 8.5. Right panel: mature biofilm grown in C-medium with initial pH of 6.5. Upper row: 45° perspective. Lower row: top view.


      Environmental signals such as low pH and critical levels of peptide pheromones initiate complex regulatory circuits leading to biofilm formation in GAS. The details in environmental triggers, transcriptional changes, and regulators involved seem to be strain-specific and are not completely understood yet.

      Antibiotic therapy

      Antibiotic choices for SSTI vary between specialties and institutions, reflecting differing patient populations, anatomical site, resistance patterns, MRSA risk (see accompanying article, p23) and local policy.

      Published guidance is deliberately non-prescriptive with respect to antibiotic choice, in part reflecting these complexities, but also because SSTI clinical trials typically exclude the most severely ill patients and are powered only to show non-inferiority between agents. 5,6

      For patients admitted to hospital requiring IV treatment — and where fully sensitive organisms are isolated or suspected and there is no history of penicillin allergy — narrow-spectrum beta-lactam antibiotics such as benzylpenicillin (for beta-haemolytic streptococci) and flucloxacillin (for both beta-haemolytic streptococci and staphylococci) remain the antibiotics of choice. It is the author’s practice to use flucloxacillin monotherapy as first-line treatment for non-allergic patients unless MRSA or polymicrobial infection is suspected following assessment (see Box 1).

      When oral therapy is indicated flucloxacillin is appropriate, and for the beta-lactam-sensitive patient erythromycin or clarithromycin, clindamycin, or doxycycline (except during pregnancy or lactation and for children) are efficacious. For patients with beta-lactam sensitivity requiring IV therapy, vancomycin or clindamycin is usually selected.

      For adults with severe SSTIs requiring IV therapy, it is the author’s practice, following administration of an initial IV dose, to use a continuous infusion of either flucloxacillin (eg, 12g/24h) or vancomycin (eg, 2g/24h), to provide the maximum time for the antibiotic to be above the minimum inhibitory concentration for the suspected organism. Therapeutic drug monitoring should be performed for patients receiving vancomycin, aiming for a random-level concentration of 10–15mg/L, with higher concentrations appropriate for patients with MRSA bacteraemia.

      For patients with necrotising or rapidly progressive infections, IV clindamycin at a dose of 900mg eighthourly is added to enhance cover against toxigenic S pyogenes. Clindamycin reduces the production of streptococcal toxic shock protein by its action on bacterial mitochondria. It is also active when beta-lactams are rendered ineffective, which occurs during the static growth phase of streptococci when penicillin binding protein production is halted.

      If polymicrobial infection is suspected the spectrum of antibiotic cover should be expanded. Typically, for infected bites co-amoxiclav (IV or oral) is appropriate. Doxycyline is a suitable oral alternative if the patient is allergic to beta-lactams. Gentamicin, vancomycin and metronidazole can be considered as alternatives, but specialist advice should be sought and therapy adjusted depending on microbiological results.

      Host Antimicrobial Peptides and Bacterial Counter Strategies

      Other important host factors are the antimicrobial peptides that are essential components of the first line of defence against pathogens [ 53]. The cathelicidin LL-37 was shown to provide protection against murine necrotic skin infections caused by S. pyogenes [ 54]. However, several pathogenic bacteria secrete factors that can degrade and inactivate antimicrobial peptides, such as the streptococcal cysteine protease SpeB [ 35, 55], and the streptococcal inhibitor of complement [ 56].

      Analyses of patient tissue biopsy specimens revealed that the active LL-37 peptide was present in all infected biopsy specimens, and its expression was positively correlated with bacterial load [ 25]. Although an up-regulation of LL-37 is expected in response to streptococcal infection [ 54, 57], such a positive correlation to bacterial load, together with the fact that there are viable bacteria in the tissue during a prolonged time, strongly implied that LL-37 in the infected tissue did not efficiently contribute to bacterial killing. Further studies revealed that this lack of antimicrobial activity was likely attributable to SpeB inactivation of LL-37 at the bacterial surface [ 25], according to the model proposed by Nyberg et al [ 35]. In this model, SpeB is entrapped by the a2-macroglobulin-GRAB complex, thereby achieving an accumulation of SpeB around the bacteria, where the biological significance of an inactivation of LL-37 will be the greatest.

      It is becoming increasingly evident that many of the antimicrobial peptides act not only as antimicrobial agents, but also as significant mediators of other biological effects, including immunomodulatory and chemotactic activities [ 53]. Considering the hyperinflammatory state of these severe tissue infections, such effects would likely exacerbate the pathological responses and worsen the disease progression. In addition, a potential effect on bacterial virulence was suggested by Gryllos et al [ 58], who reported that subinhibitory concentrations of LL-37 resulted in enhanced expression of several streptococcal virulence factors, including capsule, SpyCEP/ScpC, and IdeS.

      Immunological Methods in Microbiology

      Maria M. Plummer , Charles S. Pavia , in Methods in Microbiology , 2020

      3.1 Epidemiologic and microbiologic features

      S. pyogenes is associated with many clinical conditions including pharyngitis, scarlet fever, acute rheumatic fever and glomerulonephritis. It is a Gram-positive coccus which forms beta-haemolytic colonies when cultured on a blood agar plate. It is catalase negative—a feature which rapidly distinguishes it from the morphologically similar staphylococcal group of bacteria which are also Gram-positive but are catalase positive. It is non-motile and non-spore forming and usually occurs in chains or pairs and typically has a capsule made of hyaluronic acid. It is a facultative anaerobe and grows best on medium containing blood that has been incorporated into the agar.

      The bacteria's cell wall is made up of repeating units of N-acetylglucosamine and N-acetylmuramic acid. The identification of Group A streptococci is based on serologic reactivity of cell wall polysaccharide antigens (as part of the Lancefield group-classification scheme as originally developed by Rebecca Lancefield). The sensitivity of S. pyogenes to the anti-microbial agent, bacitracin, is useful for distinguishing it from other beta-hemolytic streptococci, such as S. agalactiae, that are frequently encountered in clinical specimens. The capsule of S. pyogenes resists phagocytosis. It expresses an M protein, and a complement C5a peptidase that degrades this chemotactic peptide. In scarlet fever, S. pyogenes secretes a phage-encoded pyrogenic exotoxin, via lysogeny, that causes fever and a rash. It is most likely that post-streptococcal associated acute rheumatic fever is caused by antistreptococcal M protein antibodies and T cells that cross-react with cardiac proteins, and which gives rise to a variety of cardiac abnormalities.

      S. pyogenes is a frequent pathogen in humans of all ages. Approximately 5–15% of normal individuals carry the organism, usually in the upper respiratory tract, without disease signs or symptoms. As part of the normal flora (now often referred to as the “microbiome”), S. pyogenes can cause a symptomatic infection primarily when there are compromised defences.

      ◗ Serodiagnosis

      Serological tests are of value in the diagnosis of AGN and rheu-matic fever. These tests detect high level of antibodies produced against many streptococcal antigens. The tests detecting anti-bodies against SLO (anti-SLO, or ASO antibodies) are most frequently used for confirming rheumatic fever and AGN. The ASO antibodies appear in serum 3–4 weeks after initial infection by S. pyogenes. A titer of more than 200 indicates streptococcal infections. Higher antibody titers are found in acute rheumatic fever, whereas they are not raised in patients with glomerulone-phritis and streptococcal pyoderma. Antibodies against other streptococcal enzymes, such as DNAase B (anti-DNase B anti-bodies), hyaluronidase (anti-hyaluronidase antibodies), and streptokinase (anti-streptokinase antibodies) are also demon-strated in S. pyogenes infections. The demonstration of antibod-ies against these antigens may prove useful in the diagnosis of streptococcal pharyngitis and pyoderma.