streptococcus pyogenes is the major causative agent for pharyngitis and tonsillitis , but may also cause , although relatively rarely , invasive infections such as meningitis , sepsis , and necrotizing fasciitis . for such infections , treatment with antimicrobial agents exhibiting superior activity against s. pyogenes must be initiated immediately . penicillin agents are representative drugs used as the agents of first choice for infections caused by hemolytic streptococci . however , many reports on the sensitivity of s. pyogenes to antimicrobial agents pertain to strains isolated from patients with pharyngitis or tonsillitis , and there are very few reports focusing on strains isolated from patients with invasive infections . moreover , most of the drugs evaluated in comparison with the penicillins in these reports are oral antimicrobial agents , and there are no reports on the efficacy of intravenous antimicrobial agents , carbapenems in particular , used for the treatment of invasive infections . in order to select appropriate antimicrobial agents for the treatment of hemolytic streptococcal infections associated with serious symptoms , the minimal inhibitory concentrations ( mics ) and minimal bactericidal concentrations ( mbcs ) of various antimicrobial agents were examined for s. pyogenes strains isolated from children with invasive infections such as sepsis . although many reports state that severe infection is associated with specific emm types , no results are available for children in japan . a total of 12 strains of s. pyogenes isolated from children who were examined for invasive infections at pediatric departments in medical institutions in hokkaido , including our hospital , during the 10-year period between july 2003 and june 2012 were included for this study . severe infection with s. pyogenes was defined as : ( 1 ) detection of s. pyogenes from originally sterile sites such as blood , cerebrospinal fluid , and puncture fluid and ( 2 ) while s. pyogenes was detected from non - sterile sites such as the skin and pharynx , no other bacteria could be assumed to be the cause and , furthermore , the pathology is not inconsistent with infection due to s. pyogenes . patients considered to have invasive infections were those with serious symptoms that were almost life - threatening , those requiring a surgical procedure such as drainage and/or debridement , and those requiring hospitalization for 2  weeks or more . the mic values of seven drugs ( penicillin g , ampicillin , cefotaxime , ceftriaxone , panipenem , meropenem , doripenem ) were measured by the broth microdilution method . for determination of the mbc , a culture suspension of 10  l was collected from the plate on which the mic was measured and applied to non - selective culture media in wells containing the drug at a concentration equal to or greater than the mic . after aerobic cultivation at 35c for 2024  h , the number of colonies were counted , and the concentration that caused a decrease in the volume of the growing bacteria by 99.9  % or more was determined as the mbc . the laboratory of molecular epidemiology for infectious agents , kitasato institute of life sciences , was requested to perform a determination of the emm types . at the laboratory , m protein - coding genes were amplified by pcr and the base sequence at the 5 terminal end of the amplified product was determined ; thereafter , that sequence of 300  bp was transmitted to the cdc reference center and the types were determined by matching with the cdc database . table  1 shows a list of patients with invasive infections caused by s. pyogenes . the patients ranged widely in age from 1  day to 15  years , with patients younger than 5  years , including three neonates , accounting for half of all the patients . the disease was sepsis in four patients , skin and soft tissue infection in three patients , retropharyngeal abscess in two patients , pneumonia plus sepsis in one patient , empyema in one patient , and pyogenic arthritis in one patient . one patient ( a 15-year - old adolescent girl ) with pneumonia plus sepsis died of shock . the most prevalent emm type was emm1 ( six strains ; 50.0  % ) , followed by emm12 ( two strains ; 16.7  % ) , while emm3 , emm4 , emm6 , and emm28 were recognized in one strain each.table  1characteristics of children with severe infection due to streptococcus pyogenes 
 case numberagetype of emm 
 diagnosisisolatedantibiotic therapyprognosis11  day6pyothoraxpleural fluidabpc 11  dayscure24  days1sepsisbloodabpc 9  dayscure37  days1sepsisbloodabpc 11  dayscure41  year12cellulitisskin swababpc 7  dayscure53  years12sepsisbloodabpc 14  dayscure63  years1sepsisbloodabpc 14  dayscure75  years28arthritissynovial fluidcez 7  dayscure8 
 5  years4retropharyngeal abscesspharyngeal swabipm / cs 2  dayscure97   years3cellulitisskin exudateipm / cs 7  dayscure1011  years1erysipelaspharyngeal swababpc 7  dayscure1111  years1retropharyngeal abscesspharyngeal swababpc 10  dayscure12 
 15  years1sepsis  +  pneumoniabloodpapm / bp one shotdead 
 abpc ampicillin , cez cefazolin , ipm / cs imipenem / cilastatin sodium , papm / bp panipenem / betamipron 
 patient was transferred to another hospital for operation 
 patient was transported in a state of shock in the evening by ambulance and was transferred to an advanced emergency medical care center after receiving basic life support and a single dose of papm / bp , but died the following morning . the patient had no underlying disease associated with increased susceptibility to infection characteristics of children with severe infection due to streptococcus pyogenes 
 
 abpc ampicillin , cez cefazolin , ipm / cs imipenem / cilastatin sodium , papm / bp panipenem / betamipron 
 patient was transferred to another hospital for operation 
 patient was transported in a state of shock in the evening by ambulance and was transferred to an advanced emergency medical care center after receiving basic life support and a single dose of papm / bp , but died the following morning . the patient had no underlying disease associated with increased susceptibility to infection figures  1 and 2 show the distributions of the mics and mbcs for each drug , and table  2 shows the mic90 and mbc90 values . the drug with the most potent antibacterial activity was doripenem , with mics and mbcs of 0.004  g / ml or less for all the strains . this drug was followed in antibacterial potency by panipenem , the mic and mbc of which were determined to be 0.004  g / ml or less and 0.008  g / ml for six strains each . for meropenem , the drugs showing divergent values of the mic and mbc were : penicillin g for five strains , ampicillin for three strains , cefotaxime for five strains , and ceftriaxone for eight strains . a comparison of the sensitivities to these four drugs revealed that penicillin g had the highest activity , followed by cefotaxime , ampicillin and ceftriaxone , in that order . 2susceptibilities of s. pyogenes isolates to carbapenemstable  2mic and mbc of s. pyogenes 
 mic rangemic90 
 mbc rangembc90 
 penicillin g0.008  ~  0.0150.0150.008  ~  0.0150.015ampicillin0.015  ~  0.030.030.015  ~  0.030.03cefotaxime0.015  ~  0.030.0150.015  ~  0.030.03ceftriaxone0.015  ~  0.030.030.03  ~  0.060.03meropenem0.0080.0080.0080.008panipenem0.004 ~  0.0080.0080.004  ~  0.0080.008doripenem0.0040.0040.0040.004 susceptibilities of streptococcus pyogenes isolates to penicillins and cephems susceptibilities of s. pyogenes isolates to carbapenems mic and mbc of s. pyogenes invasive group a streptococcal infections include sepsis , empyema , bone and joint infections , necrotizing fasciitis , and streptococcal toxic shock syndrome . according to a number of reports , the estimated incidence of invasive group a streptococcal infection in children in developed countries is 13 per 100  000 children , and the mortality is 520  % . while among all age groups , the incidence is the highest in elderly people , children younger than 5  years of age are the next most commonly affected age group . among reports on invasive group a streptococcal infections in children , mulla   from the united states reported the outcomes of 25 children ranging in age from 3  weeks to 17  years with hemolytic streptococcal infections , who were examined between 1996 and 2000 . in regard to the patients age , six children were younger than 1  year old and eight children were 14  years of age , with children younger than 5  years accounting for more than half of the cases . with regard to the presence of invasive disease , 18 patients had bacteremia and three had necrotizing fasciitis . henriet et al .   from france reported that , among 28 children examined between 2000 and 2007 , 15 had joint infections , seven had soft tissue infections , three had pneumonia , and three had toxic shock syndrome , and the median age was 2.9  years . analyzed the emm types of s. pyogenes isolated from patients with invasive group a streptococcal infections diagnosed at medical institutions in japan between 2003 and 2006 . among the 74 strains obtained , many were isolated from patients aged 6080  years , while only 11 strains ( 14.9  % ) were isolated from patients younger than 20  years of age . among the 74 strains , the most prevalent type was emm1 ( 29 strains ; 39.2  % ) , followed by emm49 ( eight strains ; 10.8  % ) and emm12 and emm28 ( five strains each ; 6.7  % ) . the emm1 type was the most prevalent , which was consistent with the results of the present study . in regard to results reported from europe and the united states , according to one report , out of 247 strains , including those isolated from pediatric patients in spain between 1998 and 2009 , the most prevalent emm type was emm1 ( 60 strains ; 24.3  % ) , followed by emm3 ( 21 strains ; 8.5  % ) and emm4 ( 14 strains ; 5.7  % ) ; these results indicate a high prevalence of the emm1 type . in germany , evaluation of 586 strains revealed that the most prevalent emm type was emm1 ( 179 strains ; 30.5  % ) , accounting for nearly one - third of all the strains , followed by emm28 ( 107 strains ; 18.3  % ) and emm3 ( 56 strains ; 9.6  % ) . similarly , emm1 was also the most commonly identified type according to other reports , indicating that invasive infections are strongly associated with the emm1 type [ 7 , 10 ] . according to recent studies [ 11 , 12 ] , emm12 is the most common among the emm types of s. pyogenes isolated from pharyngitis . however , the frequencies of emm1 were about half of those in severe infections , showing differences in distributions of emm types between pharyngitis and severe infections . while there are many reports [ 7 , 9 , 13 , 14 ] examining the associations between emm types and pathogenic factors , such as toxin production and super - antigens , why emm type 1 is commonly detected in severe infections has not been elucidated . among the drugs used in the drug sensitivity testing , doripenem exhibited the highest activity , with an mic of 0.004  g / ml against all strains . the mic90 values of panipenem and meropenem were 0.008  g / ml , indicating the superior antibacterial activity of these drugs as compared to that of other agents ( mic90 : 0.0150.03  g / ml ) . the mic and mbc of carbapenem agents were consistent for all strains , whereas those of the penicillin agents and cephem agents were not quite so consistent . no reports have focused on s. pyogenes exhibiting resistance to penicillin agents , although there are occasional reports of tolerance [ 1517 ] ; it is considered that , due to the divergence between mic and mbc , the strain requires more time to be killed as compared to other strains , or there is some kind of mechanism against killing in the bacteria . this phenomenon is considered to be one of the reasons for the recurrence of pharyngitis and tonsillitis caused by s. pyogenes , even after the administration of effective antimicrobial agents [ 18 , 19 ] . moreover , it is also considered to be one reason why not a few patients treated with penicillin alone have poor outcomes in association with the  inoculum effect ,  a decrease in drug susceptibility when the bacterial load is large in patients with severe infections . while there are no unified criteria for tolerance , it is said that the mic / mbc ratio is equal to or greater than 16 or 32 [ 19 , 21 ] . in this study , the maximum mic / mbc ratio was twofold , which did not fulfill the definition of tolerance ; however , this phenomenon seems to require attention . penicillin agents are considered as the drugs of first choice for invasive streptococcal infections , and concurrent use of clindamycin is recommended , with the expectation of its special antimicrobial activities , such as tissue permeability , inhibition of toxin production , and promotion of phagocytic activity . carbapenem agents have lower mbc than penicillin agents and , moreover , penicillin is highly effective in the log phase while its effect decreases in a steady state . however , there are results , albeit in vitro , raising the possibility of high efficacy in the early stationary phase . it is thus considered to be appropriate for the treatment of severe s. pyogenes infection . as there is no other study comparing the sensitivities of s. pyogenes to penicillin agents , carbapenem agents , and cephem agents , this awaits further basic and clinical investigations .