Streptococcus Milleri Research Articles

Microbiology of peritonsillar abscess in the South Estonian population.

ObjectiveThe first aim of this study was to compare the microbiota of different locations (pus, tonsillar fossa, blood) in peritonsillar abscess (PTA) patients in order to optimize the sampling scheme. The second aim was to estimate the occurrence of tonsillitis episodes and macroscopic oropharyngeal signs characteristic of recurrent tonsillitis in PTA patients.MethodsThe study group consisted of 22 consecutive patients with PTA undergoing bilateral tonsillectomy. The PTA was punctured; pus and tonsillar fossa biopsy samples and the peripheral blood cultures were collected. The index of tonsillitis was calculated by multiplying the number of tonsillitis episodes per year by the morbidity period in years. Macroscopic oropharyngeal signs were evaluated and they were as follows: tonsillar sclerosis, obstruction of the tonsillar crypts, scar tissue on tonsils, cryptic debris, and lymphatic tissue aggregates.ResultsThe cultures of the pus were positive in 16 out of 22 patients and the cultures of the tonsillar fossa samples were positive in all cases. In total, 62 different organisms were found from tonsillar fossa, pus, and blood samples, which belonged to 5 different phyla and 18 different families.In the tonsillar fossa, the most frequent bacteria found were Streptococcus spp. In pus samples, the most frequently found bacteria were Streptococcus spp. and bacteria from the Streptococcus milleri group.ConclusionPTA patients had mixed anaerobic and aerobic microbiota both in the tissue of the tonsillar fossa and the pus of the peritonsillar space. We demonstrated that the tonsillar fossa specimen is a better material for microbiological analyses, because it reveals more bacteria per culture. PTA patients usually have a low number of tonsillitis episodes in their previous history, but a relatively high number of macroscopic oropharyngeal signs, indicating the sclerotic process in palatal tonsils.

Streptococcus anginosus Group Bacteria: No Longer a Case of Mistaken Identity

The anginosus, or “milleri,” group of bacteria belonging to the genus Streptococcus have long been the cause of much consternation with respect to both their nomenclature and accurate laboratory identification. Although they are considered part of the normal flora of the human gastrointestinal, urogenital, and upper respiratory tracts, Streptococcus anginosus group (SAG) bacteria have the ability to cause severe invasive infections with a propensity for abscess formation. Although whole-genome and multilocus sequence analyses of SAG bacteria support the current view that the SAG is made up of three distinct species (S. anginosus, S. constellatus, and S. intermedius) and a number of subspecies, advances in molecular technology have allowed more accurate laboratory identification of SAG bacteria; it remains to be seen whether distinguishing these species and subspecies will be of clinical importance.

Streptococcus anginosus (milleri) Group Strains Isolated in Poland (1996-2012) and their Antibiotic Resistance Patterns.

Streptococcus anginosus, Streptococcus intermedius and Streptococcus constellatus form a group of related streptococcal species, namely the Streptococcus Anginosus Group (SAG). The group, previously called "milleri" had been rarely described until 1980/1990 as source of infections. Nowadays SAG bacteria are often described as pathogens causing predominantly purulent infections. The number of infections is highly underestimated, as SAG strains are often classified in the microbiology laboratory as less virulent "viridans streptococci" Epidemiological situation regarding SAG infections in Poland has been unrecognized, therefore we performed a retrospective analysis of strains isolated between 1996 and 2012. Strains suspected of belonging to SAG were re-identified using an automated biochemical approach (Vitek2) and MALDI-TOF MS. We performed first analysis of antibiotic resistance among SAG strains isolated in Poland using automated methods (Vitek2), disk diffusion tests and E-Tests. We also performed PCR detection of resistance determinants in antibiotic resistant strains. Clonal structure of analyzed strains was evaluated with PFGE and MLVF methods. All three species are difficult to distinguish using automated diagnostic methods and the same is true for automated MIC evaluation. Our analysis revealed SAG strains are rarely isolated in Poland, predominantly from purulent infections. All isolates are very diverse on the genomic level as estimated by PFGE and MLVF analyses. All analyzed strains are sensitive to penicillin, a substantial group of strains is resistant to macrolides and the majority of strains are resistant to tetracycline.

Antibiotic Treatment of Hidradenitis Suppurativa.

Although hidradenitis suppurativa (HS) is not primarily an infectious disease, antibiotics are widely used to treat HS. Recent microbiological data show that HS suppurating lesions are associated with a polymorphous anaerobic flora, including actinomycetes and milleri group streptococci, and can therefore be considered as polymicrobial soft tissue and skin infections. Analysis of the literature provides little information on the efficacy of antibiotics in HS but suggests a beneficial effect of certain antimicrobial treatments, depending on the clinical severity of the disease. Patients must be informed and should agree with the treatment strategy before starting antibiotic treatments.

扁桃周囲膿瘍から波及した食道粘膜下膿瘍の1例

We report the case a 54-year-old male patient who was admitted to our hospital with a sore throat and evidence of a high inflammatory reaction. Computed tomography revealed a peritonsillar abscess extending from the inferior right tonsil to the cervical esophagus, and thickening of the esophageal wall. The pharyngeal abscess was drained through an intraluminal incision in the mesopharynx. Culture of the abscess fluid showed growth of Streptococcus constellatus, one of the organisms of the Streptococcus milleri group. We diagnosed the patient as having phlegmonous esophagitis secondary to peritonsillar abscess. The inflammation was judged to have extended to the submucosal space of the esophagus from the pharyngeal mucosal space. Since the inflammation was contained within the esophageal wall and did not progress to mediastinitis, critical symptoms did not appear and the patient improved through conservative management with antibiotics

Purulent Pericarditis Caused by Streptococcus Milleri Strains; the Gained Experience from Nine Reported Cases

Bacterial pericarditis caused by Streptococcus Milleri Group (SMG) strains is a particularly exceptional pathology. All the eight previous reported cases were under the care of medical teams and the seven existed reports in medical journals were more or less from this perspective. Herein, we reported a unique case of a pericardial-cutaneous fistula resulting from a recurrent purulent pericardial effusion caused by SMG strains, which had been treated with open surgical drainage two months before. A thorough review of the surgical treatment options and the results has also been presented.

The clinical features of respiratory infections caused by the Streptococcus anginosus group.

BackgroundThe Streptococcus anginosus group (SAG) play important roles in respiratory infections. It is ordinarily difficult to distinguish them from contaminations as the causative pathogens of respiratory infections because they are often cultured in respiratory specimens. Therefore, it is important to understand the clinical characteristics and laboratory findings of respiratory infections caused by the SAG members. The aim of this study is to clarify the role of the SAG bacteria in respiratory infections.MethodsA total of 30 patients who were diagnosed with respiratory infections which were caused by the SAG bacteria between January 2005 and February 2015 were retrospectively evaluated.ResultsRespiratory infections caused by the SAG were mostly seen in male patients with comorbid diseases and were typically complicated with pleural effusion. Pleural effusion was observed in 22 (73.3%) patients. Empyema was observed in half of the 22 patients with pleural effusion. S. intermedius, S. constellatus and S. anginosus were detected in 16 (53.3 %), 11 (36.7 %) and 3 (10.0 %) patients, respectively. Six patients had mixed-infections. The duration from the onset of symptoms to the hospital visit was significantly longer in “lung abscess” patients than in “pneumonia” patients among the 24 patients with single infections, but not among the six patients with mixed-infection. The peripheral white blood cell counts of the “pneumonia” patients were higher than those of the “lung abscess” patients and S. intermedius was identified significantly more frequently in patients with pulmonary and pleural infections (pneumonia and lung abscess) than in patients with bacterial pleurisy only. In addition, the patients in whom S. intermedius was cultured were significantly older than those in whom S. constellatus was cultured.ConclusionsRespiratory infections caused by the SAG bacteria tended to be observed more frequently in male patients with comorbid diseases and to more frequently involve purulent formation. In addition, S. intermedius was mainly identified in elderly patients with having pulmonary infection complicated with pleural effusion, and the aspiration of oral secretions may be a risk factor in the formation of empyema thoracis associated with pneumonia due to S. intermedius.Electronic supplementary materialThe online version of this article (doi:10.1186/s12890-015-0128-6) contains supplementary material, which is available to authorized users.

Unusual Presentation of Streptococcus anginosus With Severe Sepsis, Liver Abscess Secondary to Biliary Tract Perforation

Streptococcus anginosus (S. anginosus) is a member of the anginosus group of streptococci. They are part of the normal commensal flora of human mucous membranes and are infrequent pathogens. Infections range from minor oral infections to serious, complicated, life-threatening infections. They are commonly associated with abscess formation, and bacteremia is rare. We report here a case of severe sepsis with liver abscess secondary to biliary tract perforation caused by S. anginosus in a diabetic female patient. J Med Cases. 2015;6(11):508-511 doi: http://dx.doi.org/10.14740/jmc2326w

Post extraction severe Streptococcus Milleri temporal space abscess of odontogenic origin. A case series

Post extraction severe Streptococcus Milleri temporal space abscess of odontogenic origin. A case series

Deciphering the microbiology of hidradenitis suppurativa: a step forward towards understanding an enigmatic inflammatory skin disease.

Keywords: anaerobes; hidradenitis suppurativa; inflammatory disease; microbiology; soft tissue and skin infections

A Reference Broth Microdilution Method for Dalbavancin In Vitro Susceptibility Testing of Bacteria that Grow Aerobically.

Antimicrobial susceptibility testing (AST) is performed to assess the in vitro activity of antimicrobial agents against various bacteria. The AST results, which are expressed as minimum inhibitory concentrations (MICs) are used in research for antimicrobial development and monitoring of resistance development and in the clinical setting for antimicrobial therapy guidance. Dalbavancin is a semi-synthetic lipoglycopeptide antimicrobial agent that was approved in May 2014 by the Food and Drug Administration (FDA) for the treatment of acute bacterial skin and skin structure infections caused by Gram-positive organisms. The advantage of dalbavancin over current anti-staphylococcal therapies is its long half-life, which allows for once-weekly dosing. Dalbavancin has activity against Staphylococcus aureus (including both methicillin-susceptible S. aureus [MSSA] and methicillin-resistant S. aureus [MRSA]), coagulase-negative staphylococci, Streptococcus pneumoniae, Streptococcus anginosus group, β-hemolytic streptococci and vancomycin susceptible enterococci. Similar to other recent lipoglycopeptide agents, optimization of CLSI and ISO broth susceptibility test methods includes the use of dimethyl sulfoxide (DMSO) as a solvent when preparing stock solutions and polysorbate 80 (P80) to alleviate adherence of the agent to plastic. Prior to the clinical studies and during the initial development of dalbavancin, susceptibility studies were not performed with the use of P-80 and MIC results tended to be 2-4 fold higher and similarly higher MIC results were obtained with the agar dilution susceptibility method. Dalbavancin was first included in CLSI broth microdilution methodology tables in 2005 and amended in 2006 to clarify use of DMSO and P-80. The broth microdilution (BMD) procedure shown here is specific to dalbavancin and is in accordance with the CLSI and ISO methods, with step-by-step detail and focus on the critical steps added for clarity.

A Reference Broth Microdilution Method for Dalbavancin <em>In Vitro</em> Susceptibility Testing of Bacteria that Grow Aerobically

Antimicrobial susceptibility testing (AST) is performed to assess the in vitro activity of antimicrobial agents against various bacteria. The AST results, which are expressed as minimum inhibitory concentrations (MICs) are used in research for antimicrobial development and monitoring of resistance development and in the clinical setting for antimicrobial therapy guidance. Dalbavancin is a semi-synthetic lipoglycopeptide antimicrobial agent that was approved in May 2014 by the Food and Drug Administration (FDA) for the treatment of acute bacterial skin and skin structure infections caused by Gram-positive organisms. The advantage of dalbavancin over current anti-staphylococcal therapies is its long half-life, which allows for once-weekly dosing. Dalbavancin has activity against Staphylococcus aureus (including both methicillin-susceptible S. aureus [MSSA] and methicillin-resistant S. aureus [MRSA]), coagulase-negative staphylococci, Streptococcus pneumoniae, Streptococcus anginosus group, β-hemolytic streptococci and vancomycin susceptible enterococci. Similar to other recent lipoglycopeptide agents, optimization of CLSI and ISO broth susceptibility test methods includes the use of dimethyl sulfoxide (DMSO) as a solvent when preparing stock solutions and polysorbate 80 (P80) to alleviate adherence of the agent to plastic. Prior to the clinical studies and during the initial development of dalbavancin, susceptibility studies were not performed with the use of P-80 and MIC results tended to be 2-4 fold higher and similarly higher MIC results were obtained with the agar dilution susceptibility method. Dalbavancin was first included in CLSI broth microdilution methodology tables in 2005 and amended in 2006 to clarify use of DMSO and P-80. The broth microdilution (BMD) procedure shown here is specific to dalbavancin and is in accordance with the CLSI and ISO methods, with step-by-step detail and focus on the critical steps added for clarity.

Telavancin: mechanisms of action, in vitro activity, and mechanisms of resistance.

Telavancin is a semisynthetic lipoglycopeptide derivative of vancomycin. Telavancin has a dual mechanism of antibacterial action, disrupting peptidoglycan synthesis and cell membrane function. In 2014, the Clinical and Laboratory Standards Institute (CLSI) revised the antimicrobial susceptibility testing method for telavancin, resulting in minimum inhibitory concentration (MIC) determinations that are more accurate and reproducible and demonstrate greater in vitro potency than shown with the previous testing method. The CLSI testing method changes coincided with revised telavancin MIC interpretive break point criteria for susceptibility approved by the US Food and Drug Administration for Staphylococcus aureus (≤0.12 µg/mL), Streptococcus pyogenes (≤0.12 µg/mL), Streptococcus agalactiae (≤0.12 µg/mL), Streptococcus anginosus group (≤0.06 µg/mL), and Enterococcus faecalis (vancomycin susceptible, ≤0.25 µg/mL). Telavancin is equally potent against methicillin-susceptible S. aureus (MSSA) and methicillin-resistant S. aureus (MRSA). It demonstrates activity against isolates of heterogeneous vancomycin-intermediate S. aureus and vancomycin-intermediate S. aureus but is poorly active against vancomycin-resistant S. aureus. It also demonstrates potent activity against Staphylococcus epidermidis and Streptococcus spp. (MIC90 ≤0.03 µg/mL). Thus far, it has not been possible to select for high-level telavancin resistance in the laboratory using serially passaged clinical isolates of MRSA and MSSA.

Pediatric sinogenic epidural and subdural empyema: The role of endoscopic sinus surgery

AimTo analyze the indications and outcomes of open neurosurgical approaches (ONA) and endoscopic transnasal approaches (ETA) in the surgical management of pediatric sinogenic subdural and epidural empyema. Material and methodsRetrospective single-center study design within a tertiary care referral center setting. Children less than 18 years of age consecutively operated on between January 2012 and February 2014 for drainage of a sinogenic subdural empyema (SE) or epidural (EE) empyema were included. Main outcome measures: success of first surgical procedure, persistent symptoms and sequelae at the end of the follow-up period. ResultsNine SE (53%) and 8 EE (47%) were observed. Neurological symptoms, especially seizures, were more frequent in the SE group. Perioperative pus samples were positive in 67% of the SE group and in 75% of the EE group. The most frequently isolated bacteria belonged to the Streptococcus anginosus group. CT or MR imaging showed that most empyema probably originated from the frontal sinus. However, two cases resulted from an ethmoiditis and one case from a Pott's puffy tumor, without any direct contact with the paranasal sinus. In cases of SE, the most effective surgical technique was ONA with craniotomy. Associated endoscopic sinus drainage was useful for the purpose of bacteriological diagnosis. In cases of EE, effectiveness was noted in both ONA and ETA techniques. In two cases of EE, the ETA procedure encompassed direct drainage of the empyema through the posterior wall of the frontal sinus (Draf III approach). The number of patients successfully treated after a single surgical procedure was higher in the EE group (p=0.05). Regarding outcomes, no mortalities were observed. Persistent disorders at the end of the follow-up period, especially headaches, cognitive, concentration or schooling problems, tended to be more frequent in the SE group than in the EE group (67% vs 29%), and were more commonly observed in cases requiring several surgical procedures (75% vs 12.5%) (p=0.05). DiscussionEndoscopic sinus surgery plays a critical role in the surgical management of pediatric sinogenic SE and EE. In cases of small volume EE, the endoscopic approach associated with antibiotherapy may be sufficient to treat the infectious process.

Liver abscess and respiratory disease caused by anginosus group streptococci

Anginosus group streptococci (previously known as Streptococcus milleri group) comprise three subspecies known to cause endogenous pyogenic infections. The majority of cases reported are in adults. We report a liver abscess occurring in a 3-year-old child without known preceding factors. He presented with clinical signs and X-ray findings compatible with a lower respiratory tract infection and was treated with oral amoxicillin. A blood culture grew Streptococcus constellatus and an abdominal ultra sound scan revealed a liver abscess confirmed by computerized tomography scan. Conservative management with a prolonged course of antibiotics led to complete resolution. Anginosus group streptococci can cause liver abscesses in healthy children and the presence of bacteremia due to this group of organisms should prompt investigation for occult abscesses.

In Vitro Activities of Tedizolid and Linezolid against Gram-Positive Cocci Associated with Acute Bacterial Skin and Skin Structure Infections and Pneumonia.

Tedizolid is a novel, expanded-spectrum oxazolidinone with potent activity against a wide range of Gram-positive pathogens. A total of 425 isolates of Gram-positive bacteria were obtained consecutively from patients with acute bacterial skin and skin structure infections (ABSSSIs) or pneumonia. These isolates included methicillin-susceptible Staphylococcus aureus (MSSA) (n = 100), methicillin-resistant Staphylococcus aureus (MRSA) (n = 100), Streptococcus pyogenes (n = 50), Streptococcus agalactiae (n = 50), Streptococcus anginosus group (n = 75), Enterococcus faecalis (n = 50), and vancomycin-resistant enterococci (VRE) (Enterococcus faecium) (n = 50). The MICs of tedizolid and linezolid were determined by the agar dilution method. Tedizolid exhibited better in vitro activities than linezolid against MSSA (MIC90s, 0.5 versus 2 μg/ml), MRSA (MIC90s, 0.5 versus 2 μg/ml), S. pyogenes (MIC90s, 0.5 versus 2 μg/ml), S. agalactiae (MIC90s, 0.5 versus 2 μg/ml), Streptococcus anginosus group (MIC90s, 0.5 versus 2 μg/ml), E. faecalis (MIC90s, 0.5 versus 2 μg/ml), and VRE (MIC90s, 0.5 versus 2 μg/ml). The tedizolid MICs against E. faecalis (n = 3) and VRE (n = 2) intermediate to linezolid (MICs, 4 μg/ml) were 1 μg/ml and 0.5 μg/ml, respectively. The tedizolid MIC90s against S. anginosus, S. constellatus, and S. intermedius were 0.5, 1, and 0.5 μg/ml, respectively, and the rates of susceptibility based on the U.S. FDA MIC interpretive breakpoints to the isolates were 16%, 28%, and 72%, respectively. Tedizolid exhibited 2- to 4-fold better in vitro activities than linezolid against a variety of Gram-positive cocci associated with ABSSSIs and pneumonia. The lower susceptibilities of tedizolid against isolates of S. anginosus and S. constellatus than against those of S. intermedius in Taiwan were noted.

Microbial Flora and Antibiotic Resistance in Peritonsillar Abscesses in Upstate New York.

To identify the common bacteria in recent peritonsillar abscesses and the prevalence of antibiotic resistance and compare both between adults and children. This is a retrospective chart review at a single academic institution of patients who underwent either incision and drainage or tonsillectomy for a peritonsillar abscess between 2002 and 2012 (n=69). Medical records were reviewed for cultures, comorbidities, and drainage procedures. Cultures obtained from 62.32% of peritonsillar abscesses were polymicrobial, and 34.78% were monomicrobial. The most common pathogens were β-hemolytic Streptococcus (31.88%), α-hemolytic Streptococcus (21.74%), Neisseria (14.49%), and Streptococcus milleri (13.04%). Group A β-hemolytic streptococcus was more common in children and Streptococcus milleri was more common in adults. Alpha-hemolytic streptococcus was resistant to clindamycin (6.67%) and erythromycin (6.67%). Streptococcus milleri was resistant to clindamycin (11.11%) and erythromycin (11.11%). Staphylococcus was resistant to penicillin (37.5%), oxacillin (25%), erythromycin (25%), and clindamycin (12.5%). β- and α-hemolytic Streptococci, Neisseria, and Streptococcus milleri are the most common pathogens. Streptococcus milleri is more common in adults, and β-hemolytic streptococcus is more common in children. Resistance to clindamycin and erythromycin is common in Streptococci and Staphylococci, and penicillin resistance is common in Staphylococci.

Peritoneal fluid culture and antibiotic treatment in patients with perforated appendicitis in a Pacific Island

Little data on the usefulness of microbiological samples in appendicular peritonitis (AP) are available. The objectives of the study were to document the clinical value of systematic peritoneal swabbing in complicated appendicitis, to establish whether they influence postoperative outcome, and to help to optimize empirical preoperative treatment. Charts of all consecutive patients undergoing appendicectomy for acute perforated appendicitis were analyzed over a 7-year period from 2005 to 2011 in a tertiary hospital in New Caledonia. From prospectively recorded data, microbiological culture results, and sensitivity of isolates were obtained in relation to histopathological findings and infective morbidity. Overall, 144 cases including 47 (33%) children and 97 (67%) adults with perforated appendicitis were included in the study. Fifty-one (35%) had generalized peritonitis treated laparoscopically in 30 (59%) cases. Peritoneal fluid samples yielded a positive culture in 104 (74%) patients. The most commonly recovered species were Escherichia coli (81%), Streptococcus milleri group (12%), and Pseudomonas aeruginosa (12%). Postoperative infectious complications occurred in 32 (22%) including intra-abdominal abscess (n=20) and wound infection (n=12). Probabilist antibiotic regimen was less often suitable in children than in adults (p=0.003). Infectious complications occurred more often in cases with an unsuitable antibiotic regimen after culture results compared with a suitable antibiotic regimen before culture results (p=0.01). Although antibiotic use may be considered as an adjunct to surgical intervention of AP, the appropriate use of preoperative antibiotics and modifications according to culture results are essential to prevent infectious complications.

Management and outcomes in children with sinogenic intracranial abscesses

ObjectiveTo report our experience of sinogenic intracranial abscesses in the paediatric population and to guide medical and surgical management. MethodsAll children with sinogenic intracranial abscesses presenting to a large university teaching hospital over a five-year period were included in the study. Data on clinical presentation, radiological findings, microbiology, medical and surgical management and follow-up were recorded and analysed. ResultsWe identified 27 children aged 12.9±3.4 years of which 56% were male. Fourteen (52%) children had extradural abscesses, nine (33%) subdural abscesses and four (15%) parenchymal abscesses. Early sinus drainage procedures were performed on 24 (89%) patients, and the same number required neurosurgical drainage. Streptococcus milleri was isolated in 18 (67%) cases. An initial conservative neurosurgical approach failed in 50% of cases where trialled, and was associated with longer length of stay (p=0.025). In comparison to extradural abscesses, subdural abscesses were more likely to present with neurological deficits (p<0.001) and reduced consciousness (p=0.018), and required multiple neurosurgical procedures (p<0.001), longer stays (p=0.017), and had greater morbidity at six months (p=0.017). A third of children had significant morbidity at six months, which included cognitive and behavioural problems (25%), residual hemiparesis (19%) and expressive dysphasia (7%). There were no mortalities. ConclusionSinusitis complicated by intracranial abscess remains a contemporary problem. We demonstrate good outcomes with an early combined rhinological and neurosurgical approach. S. milleri is identified as the causative organism in the majority of cases, and empirical antimicrobial treatments should reflect this.

Environment and colonisation sequence are key parameters driving cooperation and competition between Pseudomonas aeruginosa cystic fibrosis strains and oral commensal streptococci.

Cystic fibrosis (CF) patient airways harbour diverse microbial consortia that, in addition to the recognized principal pathogen Pseudomonas aeruginosa, include other bacteria commonly regarded as commensals. The latter include the oral (viridans) streptococci, which recent evidence indicates play an active role during infection of this environmentally diverse niche. As the interactions between inhabitants of the CF airway can potentially alter disease progression, it is important to identify key cooperators/competitors and environmental influences if therapeutic intervention is to be improved and pulmonary decline arrested. Importantly, we recently showed that virulence of the P. aeruginosa Liverpool Epidemic Strain (LES) could be potentiated by the Anginosus-group of streptococci (AGS). In the present study we explored the relationships between other viridans streptococci (Streptococcus oralis, Streptococcus mitis, Streptococcus gordonii and Streptococcus sanguinis) and the LES and observed that co-culture outcome was dependent upon inoculation sequence and environment. All four streptococcal species were shown to potentiate LES virulence factor production in co-culture biofilms. However, in the case of S. oralis interactions were environmentally determined; in air cooperation within a high cell density co-culture biofilm occurred together with stimulation of LES virulence factor production, while in an atmosphere containing added CO2 this species became a competitor antagonising LES growth through hydrogen peroxide (H2O2) production, significantly altering biofilm population dynamics and appearance. Streptococcus mitis, S. gordonii and S. sanguinis were also capable of H2O2 mediated inhibition of P. aeruginosa growth, but this was only visible when inoculated as a primary coloniser prior to introduction of the LES. Therefore, these observations, which are made in conditions relevant to the biology of CF disease pathogenesis, show that the pathogenic and colonisation potential of P. aeruginosa isolates can be modulated positively and negatively by the presence of oral commensal streptococci.