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propionibacterium acnes is a gram - positive , slow - growing , anaerobic bacillus and a part of the normal flora of the skin , nasopharynx , oral cavity , and gastrointestinal and genitourinary tracts .
although p. acnes is a frequent contaminant of the cultures obtained percutaneously , it has been implicated as the cause of central nervous system ( cns ) infections such as brain abscesses and subdural and epidural empyemas .
most reported cases of p. acnes cns infections occur in association with shunt infections , and these infections are rarely found in the absence of infected hardware . in korea
, one study reported a case of p. acnes cns infection in a patient with chronic subdural hematoma who presented with subdural empyema and had not undergone neurosurgery .
the identification of p. acnes as the causative organism may be difficult because of its slow growth , which makes rapid and accurate identification an important consideration in differential diagnosis . here
, we describe a case of large cerebral abscess caused by p. acnes 13 months after neurosurgery and confirmed by 16s rrna gene sequencing .
a 70-yr - old man with pus discharge from a postoperative wound was admitted to the seoul national university hospital .
the patient had undergone decompressive craniectomy and partial lobectomy 13 months before admission because of cerebral infarction and subsequent brain swelling . during rehabilitation , at 10 months after the neurosurgery ,
three days before admission , the patient showed pus discharge from the wound and started to experience pain .
initial vital signs were as follows : blood pressure , 128/84 mmhg ; pulse rate , 50/min ; respiratory rate , 18/min ; and body temperature , 36.0. initial laboratory test results were as follows : hb , 12.9 g / dl ; white blood cell count , 7.310/l ( neutrophils , 53% ; lymphocytes , 34% ; monocytes , 9% ; eosinophils , 3% ; and basophils , 1% ) ; platelets , 18910/l ; and c - reactive protein level , 0.17 mg / dl .
results of the liver function tests and creatinine and glucose levels were within reference ranges .
brain magnetic resonance imaging revealed fluid collection , with peripheral enhancement in the frontotemporal area and diffuse restriction suggestive of brain abscess .
after removal of the bone flap , yellowish pus was expelled , and large amounts of purulent materials were found at the subdural area .
the pus was cultured under anaerobic conditions ( 80:10:10 atmosphere of n2:h2:co2 at 37 ) by using the macs mg500 anaerobic workstation ( don whitley scientific , west yorkshire , uk ) .
therefore , we performed 16s rrna gene sequencing with the initial pus specimen by using microseq 500 16s rrna bacterial identification kits ( pn 4346298 ; applied biosystems , foster city , ca , usa ) , according to the manufacturer 's instructions .
the sequence was analyzed using the microseq i d 16s rrna 500 library v2.1 and analysis software v2.0 ( applied biosystems ) .
the isolate was identified as p. acnes ( 100.00% match ) according to the identity algorithm of the clsi guideline mm18-a on gram - positive anaerobes .
nine days after the culture of the initial pus , circular , convex , and opaque colonies with a diameter of 1 - 2 mm were observed on brucella agar , and the organisms were found to be gram - positive rods ( fig .
the api rapid i d 32a test ( biomrieux , marcy - l'etoile , france ) was performed using a colony ; however , the result revealed an unacceptable profile for p. acnes ( profile 2103771604 ) ( table 1 ) .
it indicated positive activity of alkaline phosphatase and phenylalanine arylamidase , which are not usually observed in p. acnes .
in addition , the pus specimen was transferred to the enrichment broths of the bactec fx ( becton dickinson , sparks , md , usa ) and bact / alert ( biomrieux , organon teknika , durham , nc , usa ) systems , and growth was obtained only in the bactec fx system after incubation for 5 days .
api rapid i d 32a test was also performed using a colony from brucella agar , and the organism was identified as p. acnes ( profile 2103770604 , % i d : 99.9 , t index : 0.43 ) . at this time , phenylalanine arylamidase activity was found to be absent .
therefore , p. acnes was thought to be the causative organism for the purulent pus and large brain abscess .
the patient received intravenous vancomycin and ceftazidime treatment for 2 weeks and was then discharged .
p. acnes is likely to cause infection after intracranial operative procedures because of its prevalence in the scalp flora , where it resides in the anaerobic environment of the hair follicle . in a retrospective review of the postoperative courses of 2,111 patients who underwent neurosurgical procedures ,
the incidence of postoperative cns infections ( pcnsis ) caused by p. acnes ( 4 cases ; 25% of the infections ) was second only to that of infections caused by staphylococcus aureus ( 8 cases ; 50% of the infections ) .
although , in the above - mentioned study , the pcnsis caused by p. acnes were associated with ventriculoperitoneal shunts and ommaya reservoir placement , cases involving intracranial operations have been well described in other studies [ 2 , 6 , 8 , 9 ] .
p. acnes is often not readily identified as a cause of pcnsi because the signs of infection are observed only after a delay and because p. acnes has a slow - growing nature .
our finding of brain abscess caused by p. acnes , which appeared 13 months after neurosurgery , was consistent with previous findings ( 21 days to 10 yr ) .
furthermore , colonial growth was first observed 9 days after inoculating the pus on brucella agar , as expected from previous reports ( 2 - 9 days , median : 4 days ) .
the results of the api rapid i d 32a test performed using the colonies obtained after direct pus inoculation and those from brucella agar , which was inoculated with the microbial growth in the bactec enrichment broth , were somewhat discrepant .
this can be explained on the basis of previous findings . in a previous study on the exoenzymes of p. acnes , it was observed that the activity of many of the enzymes of p. acnes appeared to be dependent on the culture substrate . in that study , the researchers used the api zym system ( analytab products inc . ,
plainview , ny , usa ) , which is the predecessor of the atb 32a system ( now renamed as rapid i d 32a ) . regarding the exoenzymes of the 30 strains of p. acnes grown in 6 different media , including 5% sheep blood agar , the percentages of the strains exhibiting strong enzymatic activity varied depending on the type of medium .
for example , 80% of the tested p. acnes strains exhibited -galactosidase activity in thioglycollate medium , but only 10% of the strains exhibited -galactosidase activity in 5% sheep blood agar .
moreover , a study evaluating the use of the atb 32a system for identifying anaerobic bacteria isolated from clinical specimens supported this finding .
this study , which was performed using colonies grown on columbia sheep blood agar , revealed that some strains of anaerobic bacteria , including propionibacterium spp .
, showed false - positive or false - negative results for various enzymes , leading to unacceptable profiles or misidentification of the species .
one of the 11 tested p. acnes , atcc 6919 , showed false - positive -glucosidase activity , weak -n - acetyl - glucosaminidase activity , and a false - negative nitrate reduction test result , thereby presenting an unacceptable profile . in this study , the separation index of 260 strains of anaerobic bacteria was calculated .
the result showed that the separation index of alkaline phosphatase was 2-fold higher than that of phenylalanine arylamidase ( 1,440 vs. 704 ) , indicating that the discriminatory power of alkaline phosphatase for the identification p. acnes is higher than that of phenylalanine arylamidase .
however , in our case , the ability to identify p. acnes was not dependent on the negative results of alkaline phosphatase activity ; in fact , the negative results of phenylalanine arylamidase activity were important for profiling p. acnes .
we assume that this is because the colony we used in the api test was selected from those growing on brucella agar , and the substrate in the brucella agar might have affected the results of the enzyme activity analysis . after transferring the pus into the enrichment broth of both bactec and bact / alert systems , we observed growth only in the bactec system after 5 days of incubation . the time - to - positivity ( ttp ) of p. acnes
was studied using only the bactec 9240 system ( 56.819.5 hr ) , and there were no comparative data of the ttp of the 2 systems .
earlier , studies on identifying the recovered date were performed using both systems . in a previous study ,
the bactec 9240 system was used to study 80 culture specimens recovered as propionibacterium spp . , out of which 48 ( 60.0% )
after 120 hr , 66 ( 82.5% ) were identified as positive after 144 hr , and 80 ( 100.0% ) were identified as positive after 192 hr . however , in another study , when the bact / alert system was used to evaluate 198 positive cultures , the cumulative percentage of the positive cultures was 23% after 120 hr , 63% after 144 hr , and 100% after 168 hr .
therefore , with an incubation time of 5 days , the propionibacterium spp . would go undetected in 77% of the positive samples if the bact / alert system was used and in 40% of the positive samples if the bactec 9240 system was used .
the reason for the prolonged detection time in the bact / alert system , in comparison to that in the bactec system , has not been explained .
the factors that might affect ttp are the composition of the media , the texture of the culture bottle , and the detection system itself .
however , the enrichment broths of both systems are not significantly different and contain soybean - casein digest with vitamins , amino acids , and carbohydrates .
further , although the culture bottles in the bactec system and the bact / alert system were made of glass and plastic , respectively , a recent study concluded that the performance of the newly invented plastic bottles was similar to that of the glass bottles used in the bact / alert system .
therefore , the difference in detection times is possibly because of the computer algorithm modifications in each system . in our case
, we did not attempt to detect p. acnes using the bact / alert system after 7 days of incubation .
however , we believe that if the incubation had been carried out for a longer period , the bact / alert system may have detected the organism . pcr amplification and sequencing of the 16s rrna gene have been used successfully in previous studies to detect the slow - growing p. acnes in vascular prosthesis infections and infective heart valve endocarditis [ 16 , 17 ] . in these cases ,
no bacteria were observed in the gram stain , and no growth was observed after culturing .
furthermore , a recent study revealed that 16s rrna gene sequencing performed simultaneously with the conventional identification methods of detecting anaerobic bacteria from blood cultures offered benefits over the conventional methods of identification .
they reported improvement in the percentage of identification ( up to the species level ) and in the detection time .
this combined method might be superior to culture methods in the diagnosis of bacterial infections if the patient is receiving empirical antibiotics , if the infection is caused by bacteria with unusual growth requirements , or if the pathogen is of an unexpected strain . although we could not determine the minimum inhibitory concentration , p. acnes is susceptible to many antibiotics , including the majority of the -lactams , clindamycin , vancomycin , and fluoroquinolones .
the p. acnes isolate obtained from our patient was -lactamase negative , and the infection was resolved after administering vancomycin and ceftazidime . in summary , using conventional methods with the api rapid i d 32a kit or enrichment broths , and especially using only the bact / alert system , could result in misidentification , no identification , or delayed identification of p. acnes . in our case ,
had we not performed 16s rrna gene sequencing , the detection time , workload , and the costs of identifying the correct pathogen would have increased . therefore
, 16s rrna gene sequencing could be a useful alternative method for the rapid and timely detection of p. acnes in pcnsis .
propionibacterium acnes is a gram - positive anaerobic bacillus and a normal inhabitant of the skin . 
 although it is often considered a contaminant of blood cultures , it can occasionally cause serious infections , including postoperative central nervous system infections . here , we report the case of a 70-yr - old man who developed a large cerebral abscess caused by p. acnes 13 months after neurosurgery . immediate gram staining of the pus from his brain revealed the presence of gram - positive coccobacilli . however , colony growth was observed only after 5 days of culture . 
 therefore , we performed 16s rrna gene sequencing of the pus specimen . 
 the isolate was identified as p. acnes . 
 the colonies developed 9 days after the initial culture . 
 the api rapid i d 32a test ( biomrieux , france ) was performed using a colony , but an unacceptable profile was obtained . 
 then , the pus was transferred into the enrichment broths of the bactec fx ( becton dickinson , usa ) and bact / alert 3d ( biomrieux , organon teknika , usa ) systems , but only the bactec fx system could detect growth after 5 days . 
 we performed 16s rrna gene sequencing and api rapid 32a profiling with a colony recovered from brucella agar , which was inoculated with the microbial growth in the enrichment broth from the bactec fx system . 
 the organism was identified as p. acnes by both methods . 
 this case suggests that 16s rrna gene sequencing may be a useful alternative for identifying slowly growing p. acnes from specimens that do not show growth after 5 days of culture .