Chronic periprosthetic joint infection (PJI) is a devastating complication of hip and knee arthroplasty that leads to a significant increase in joint loss of function, morbidity, and mortality. The traditional assumption is that chronic PJI can be defined as an infection in a prosthetic joint that occurs greater than 4 weeks from the index procedure or, according to the Centers for Disease Control and Prevention (CDC), greater than 90 days after the patient’s index procedure. However, following the recommendations from the last International Consensus Meeting (ICM) on PJI, it is better to understand a chronic PJI as a continuum that leads to the establishment of biofilm. Chronic PJI can also occur as a result of late hematogenous spread in which duration of symptoms is variable and does not necessarily have an established time frame. The key to setting the goals of treatment in chronic PJI is to rely on understanding bacterial biofilms and the pathogenic mechanisms that make them resistant to regular antibiotic treatment and allow them to invade the implant, its bone interface and surrounding tissues, and how host and bacterial factors may affect treatment success. This has led to the coupling of surgical interventions with local and systemic antibiotic therapy to mechanically or chemically disrupt the biofilm and allow antibiotic therapy to further eradicate residual infection in the joint, including the soft tissues and the bone. The surgical treatment options available with the goal of controlling infection include debridement and implant retention (DAIR), and one-stage and two-stage procedures. The choice of treatment depends on the bacterial factors, host factors, and condition of the extremity in terms of the soft-tissue coverage and bone stock. Furthermore, targeted antibiotic therapy, based on cultures and antibiogram to known organisms, significantly improves treatment success. However, despite advances in our understanding of chronic PJI, pathophysiology, and advances in surgical techniques to address chronic infection, treatment failure remains common; a recent study has reported failure rates as high as 36%. Moreover, the 5-year mortality rate after acquiring PJI is about 21.12%, which is higher than some of the most common cancers. In failed cases, resection arthroplasty, amputation, and fusion procedures must be discussed with the patient. Chronic suppression with antibiotics is necessary in some selected cases, as we will discuss further. There are two types of factors and their grading, in addition to the type of bacteria that would guide the decision-making processes and the current available treatment methods for chronic PJI, that will be described in this book chapter: systemic host grade and local extremity grade.
Systemic Host Grade
Consideration of host factors and the condition of the patient’s extremity are integral to treatment success or failure in chronic PJI and can also help guide treatment selection. Systemic host compromising factors enlisted by McPherson et al. are as follows:
Age ≥80 years
Chronic active dermatitis or cellulitis
Chronic indwelling catheter
Chronic malnutrition (albumin ≤3 g/dL)
Current nicotine use (inhalational or oral)
Diabetes (requiring oral agents and/or insulin)
Hepatic insufficiency (cirrhosis)
Immunosuppressive drugs (methotrexate, prednisone, cyclosporine, and the like)
Malignancy (history or active)
Pulmonary insufficiency (room air arterial blood gas O 2 <60%)
Renal failure requiring dialysis
Systemic inflammatory disease (rheumatoid arthritis, systemic lupus erythematosus)
Systemic immunocompromise from infection or disease
Host grade can be classified as A, B, or C based on the number of risk factors a patient has for potential treatment failure ( Table 28.1 ).
|Category||Systemic Host Grade||Local Extremity Grade|
|B—Compromised (1–2 compromising factors)||2—Compromised (1–2 compromising factors)|
|C—Significant compromise (>2 factors) or 1 of the following:||3—Significant compromise (>2 compromising factors)|
Some modifiable host factors that must be addressed prior to initiating surgical treatment for chronic PJI include uncontrolled diabetes, low host neutrophil count (<1000 absolute neutrophil count [ANC]), CD4+ T cell count <100, active intravenous drug abuse, active hepatitis C virus, poor dentition, nicotine use, malnutrition with low serum albumin and prealbumin levels, chronic active infection at another site or joint, and neoplasm or inflammatory condition under treatment that compromises the host’s immune system. There are some mental conditions such anxiety and depression, or chronic use of narcotics that should be addressed as well if possible. Failure to address these factors leads to a significant increase in treatment failure regardless of the treatment chosen.
Local Extremity Grade
The local extremity or wound factors that must be taken into consideration when deciding what type of treatment to administer include soft-tissue loss from the active infection, prior debridements or trauma, large subcutaneous abscesses, prior periarticular fracture, prior local irradiation to wound areas, and vascular insufficiency (see Table 28.1 ). Local extremity (wound) compromising factors are as follows:
Active infection present >3 to 4 months
Multiple incisions (creating skin bridges)
Soft-tissue loss from prior trauma
Subcutaneous abscess >8 cm 2
Synovial cutaneous fistula
Prior periarticular fracture or trauma about the joint (especially crush injury)
Prior local irradiation to wound area
Vascular insufficiency to extremity (absent extremity pulses, chronic venous stasis disease, significant calcific arterial disease)
By taking a systematic approach to classifying host and extremity grade, a treatment algorithm can be selected, which has been developed by the senior author (CAH, Table 28.2 ).
|Antibiotic suppression alone||Host C with Extremity 1|
|I&D/DAIR||Host A or B with Extremity 1|
|1-stage revision||Host A or B with Extremity 1|
|2-stage revision||Any host with any extremity|
|Arthrodesis||Host A or B with Extremity 1|
|Amputation/Disarticulation||Any host with Extremity 3|
PJI should be suspected in any patient with either a consistently painful total joint (hip or knee) or an acute onset of pain in a once pain-free hip or knee. The Musculoskeletal Infection Society (MSIS) criteria used to diagnose PJI, which was slightly modified in 2013, has been shown to have a sensitivity and specificity of 86.9% and 99.5%, respectively. In addition to the full history, physical examination, documentation of fixation state on radiographs, and data collection for the MSIS criteria, synovial fluid aspiration biomarkers such as alpha-defensin and leukocyte esterase have been shown to be very useful and accurate in immunosuppressed patients and patients using antibiotics at the time of joint aspiration. Revised diagnostic criteria were introduced during the last ICM in 2018 that have improved sensitivity and specificity when combined with alpha-defensin and D-dimer as adjuvant biomarkers. The main base of diagnosis of PJI is joint aspiration. In cases in which synovial fluid has not been obtained or the results are inconclusive, the next step is to repeat the aspiration. , Antibiotic-resistant bacteria—i.e., methicillin-resistant Staphylococcus aureu s (MRSA)—increase the chances of failure when treating PJI, especially when DAIR is used as the main treatment. Once the host and extremity grades have been established and the type of bacteria that is causing the infection has been identified, the most appropriate type of treatment will be chosen (see Table 28.2 ).
One-Stage Revision Arthroplasty
While two-stage arthroplasty is still the gold standard for the treatment of chronic PJI in the United States, this may not always be the only choice of management. The inherent morbidity associated with two surgeries versus one, along with a 26% 5-year mortality with two-stage revisions, should caution the surgeon to consider the causing microorganisms, soft-tissue condition, bone stock, host factors, prior treatments, and chronicity of infection when making the decision. Moreover, while the literature is replete with investigations showing success rates between 75% and 100% in two-stage revision, the “true” success rate, with no related death, no infection, no mechanical failure, or no reoperation, is about 65%. Admittedly, PJI is a multifactorial problem in which the treatment decision should be on a case-by-case basis. However, recent literature has broadly demonstrated that rates of reinfection between one- and two-stage revisions were comparable, with a trend toward better functional outcomes in the single-stage group. , Correct decision-making is imperative, as the success of repeated revision arthroplasty depreciates significantly (<60%). , ,
In one of the most recent investigations, Haddad et al. compared one-stage with two-stage revision in 102 patients with chronic knee PJI. According to their protocol, one-stage revision was indicated if there was minimal to moderate bone loss, absence of immunocompromising conditions, healthy soft tissues (extremity grade 1), and a known organism with known sensitivities for which appropriate antibiotics were available to be added to the cement at the time of prosthesis reimplantation and for oral chronic suppression. While 5 two-stage patients developed reinfection, none of the one-stage patients had infection recurrence at a minimum follow-up of 3 years. Superior Knee Society scores were found in one-stage compared with two-stage revisions. The authors recommended the indication of one-stage versus two-stage revision based on strict selection criteria. It is important to recognize the limitations of this study as both cohorts were necessarily similar, as the two-stage cohort had more aseptic revisions prior to the index procedure, and probably the complexity of the surgery may have been higher, in addition to the lack of randomization.
We recommend one-stage arthroplasty to be considered in patients who meet the following criteria: absence of sinus tract, good available soft tissue for wound closure, appropriate bone stock that does not need a tumor prosthesis for reconstruction, a favorable identifiable organism to help tailor antibiotics in the cement when cement can be used during reimplantation, an antibiotic profile that will be well tolerated by the patient for systemic use, and an organism that may be suppressed with oral antibiotics, ideally for at least 3 months.
One-stage exchange arthroplasty involves removal of the implants, performing a thorough radical debridement, and placing new implants, all during the same surgery. It is imperative to debride any infected and necrotic bone and soft tissue, pseudomembrane, prior scar, or devitalized soft tissue. An osteotomy may be required to remove a well-fixed implant; however, one must be mindful of potential complications from this. Once all of the purulence, prosthetic components, residual cement, and necrotic soft tissue and bone have been removed, a proper disruption of the remaining biofilm should be performed. This can be done mechanically with a brush or sponge; all exposed tissues should be brushed and then irrigated chemically with antiseptic solution, such as diluted povidone iodine, chlorhexidine gluconate, hydrogen peroxide, or acetic acid, to name some ( Table 28.3 ) . Accurate identification of the causative pathogen preoperatively is one key aspect of a successful one-stage exchange arthroplasty, as this will allow for an appropriate antibiotic regimen locally (mixed in the cement) as well as systemically postoperatively. Cultures and tissue samples should be obtained intraoperatively for confirmation of the causative pathogen. It is recommended to collect a minimum of 3 intraoperative sample cultures and no more than 5. Holding prophylactic antibiotics prior to incision to attempt increasing the likelihood of obtaining positive cultures is an inappropriate practice. It has been shown that this may increase the chances of having a concomitant infection and does not increase the likelihood of obtaining positive cultures. Once the extended debridement and irrigation with antiseptic solutions have been done, in addition to the building, trialing, and removal of the trial prosthetic components, the wound should be closed in 1 to 2 layers with a running stitch. A sterile protective dressing (usually a povidone iodine antimicrobial incision drape with a sponge) should be applied. All drapes need to be removed; then, the extremity should be cleaned and prepped again. The surgical team should re-scrub and change gowns and gloves. The extremity should be draped again with new drapes and new surgical instruments should be used for the reimplantation of the new prosthetic components. We usually set up “clean and dirty” surgical tables. We have a “dirty” table with all of the instruments needed to obtain exposure, do an extensive debridement, and remove the prosthetic components in addition to the cement, reamers and saws, and prosthetic trials. Our “clean” table has the instruments needed for the reimplantation, including the preparation, mixing and application of the cement, and final fixation of the prosthetic components. This setup decreases the bioburden of the operative table.
|Efficacy of Antiseptics||Protocol||Bacteria||Mycobacteria||Spores||Fungi||Effect on Biofilm|
|Acetic acid||0.19% vol/vol: 20 min soak a||Bactericidal||–||–||Fungicidal||Some|
|Chlorhexidine gluconate||0.05% in sterile water a||Bacteriostatic||Sporostatic||–||Some|
|Hydrogen peroxide||3% 50:50 dilution for 3 min b||–||Fungicidal||Limited|
|Sodium hypochlorite||0.5% c||–||Sporocidal||–||None|
|Povidone iodine||0.35% for 3 min a||Bactericidal||Fungicidal||Limited|
Final wound closure should be meticulously done, avoiding drains if possible to prevent leakage of eluded antibiotics and liberally using incisional vacuum-assisted closure (VAC) dressings to decrease wound complications. We believe that one of the key factors in the success of one-stage procedures is the use of cemented components that again allows the administration of tailored antibiotics. Hybrid fixation or cementation of the components allows a high local delivery of antibiotics. In cases in which cementless components are used, such as total hip arthroplasty (THA), calcium phosphate beads with mixed antibiotics can be used to deliver local antibiotics. In terms of outcomes of one-stage procedures, Singer et al. demonstrated a success rate of 95% in THA, excluding patients with MRSA. Success rate as high as 100% was recently reported in single-stage hip and knee exchange for PJI.
Two-Stage Revision Arthroplasty
Two-stage exchange arthroplasty has been the gold standard in the United States for the treatment of chronic PJI since 1983. This treatment method represents two surgeries, with a variable time interval between them. The first stage involves the removal of implants, a thorough irrigation and debridement, and insertion of either an articulating or static antibiotic-eluding cement spacer. An added benefit of having a “first” stage is the ability to obtain intraoperative samples for cultures. After confirmation of infection control, the second surgery involves removal of the cement spacer and the reimplantation of new components. Between the two stages, antibiotics are administered for an average of 6 to 8 weeks. Two weeks before the second stage, the antibiotics are usually stopped (antibiotic holiday) in order to monitor serologic markers such as erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP), and attempt to obtain synovial fluid with joint aspiration to evaluate for markers of infection, including white blood cell counts and cultures. In two-stage exchange arthroplasty, it is essential to confirm the control of infection before reimplantation. Unfortunately, the 2013 ICM definition (modified MSIS criteria) was not designed or validated for this purpose. Before reimplantation, and with success or failure determined at a minimum of 1-year follow-up, it has demonstrated a limited value in screening for such control alone or when combined with frozen sections (sensitivity 0%–25%). , Despite the poor screening capabilities, the combination of MSIS criteria and frozen sections is recommended at the time of reimplantation because of its high specificity. , In view of the absence of a gold standard for confirming infection control, many biomarkers have been tested. One of the most studied biomarkers, alpha-defensin, has not been reliable in confirming infection control before reimplantation, either alone or in combination with a preoperative aspiration, as its concentration values in joints with cement spacers have not been validated for diagnostic purposes. , As of this writing, there is no single accurate definitive test or criteria for the diagnosis of infection control during PJI, which serves as a foundation for the surgical decision-making process for the time of reimplantation. Different combinations of tests, along with a multidisciplinary approach, are used in clinical settings to facilitate decision-making. Likewise, the utility of a preoperative joint aspiration prior to reimplantation is debatable, as some studies have shown poor diagnostic characteristics given the fact that the amount of fluid present in a joint with a cement spacer is usually low and the white blood cell thresholds are lower than those validated for acute or chronic infections. However, the authors are still using pre-reimplantation joint aspirations to enhance possible culture identification, rule out purulence prior to the procedure, and prepare more effectively if a cement spacer exchange is needed.
While one-stage exchange arthroplasty may be the better choice for selected patients with PJI due to a known low-virulent organism with known antibiotic sensitivities, , patients who have a more virulent strain, resistant bacteria, soft-tissue/bone stock deficiencies, and so forth, may be better suited for a two-stage exchange arthroplasty. “Difficult-to-treat” bacteria such as MRSA, polymicrobial infections, Enterococcus , or Gram-negative bacteria tend to have worse outcomes and lower infection control success rates, and have been shown to be independent predictors of treatment failure. Even though their reinfections were mostly new bacteria (90%), Rossmann et al. showed a 37.5% revision rate due to reinfection and an overall survival rate of only 45% when doing a one-stage exchange arthroplasty in patients with confirmed Enterococcus PJI. While difficult-to-treat bacteria have worse outcomes compared with those that are not difficult to treat, this failure rate was still higher than most other recent publications looking at highly virulent PJI treated with the two-stage protocol. The two-stage exchange arthroplasty, which allows for repeat debridement and tissue sampling, may be more appropriate for these cases.
Disadvantages of two-stage arthroplasty include increased cost, cement spacer morbidity with decreased quality of life, joint contractures, disuse osteopenia, and muscle atrophy. Such adverse outcomes should always be discussed with the patient before embarking on such treatment.
The technique for two-stage arthroplasty includes aggressive debridement, with removal of sinus tract, scar tissue, infected or devitalized bone or soft tissue, and removal of all implants/cement. We recommend thorough debridement and reaming of intramedullary canals and use of adjunctive chemical biofilm disruptor solutions with antiseptic properties (see one-stage section). If the sinus tract involves the extensor mechanism or patella, this also must be debrided and removed. Full patellectomy should be avoided. In those cases that need patellectomy, we recommend leaving the anterior aspect of the cortex and periosteum adjacent to the extensor mechanism to prevent more soft-tissue damage. Placement of an antibiotic spacer should occur only after a complete change of instruments with a clean setup. We recommend the use of 3.6 g tobramycin and 3 g vancomycin per bag of cement based on their synergistic antimicrobial action and increased elution properties. High-viscosity cement should also be considered as it has better elution properties. However, if this cement type is used, a lower concentration of antibiotics should be used (2.4 g tobramycin and 2 g vancomycin per bag of cement) to avoid potential high concentrations in blood, which can be nephrotoxic. An antibiotic powder mix should be added to the cement 30 seconds after the cement has been mixed to increase formation of powder crumbs, porosity, and increase the final elution. The type of cement spacer used depends on the bone stock and soft tissues available. If there is significant bone loss (usually, the metaphysis is compromised and there is no stable apposition surface for a dynamic spacer), a static spacer is commonly constructed. Otherwise, we try to use dynamic spacers to preserve some joint function and make the reimplantation surgery easier. We usually try to use a dynamic spacer on both the hip and knee. ,
Free or rotational flaps may be needed to obtain wound closure and should be considered based on the patient’s soft-tissue loss. If there is more than 2 cm of skin loss mainly on the distal aspect of the wound, this will limit soft-tissue closure after debridement and may indicate the need for additional soft-tissue reconstruction, such as muscle flap and skin autograft. This should be coordinated with plastic reconstructive surgery prior to the procedure to avoid delays in the operating theater. We suggest performing soft-tissue coverage during the first stage. When the second stage is performed, the plastic surgery team should elevate the muscle flap and be available for the closure after the reimplantation.
For both stages, a “clean and dirty” setup should be used in the OR. For the first stage, the cement spacer should be placed on a “clean” table. To fully take advantage of the two procedures involved during a two-stage revision, when the reimplantation is done, the removal of the spacer and debridement should be done with a “dirty” table. Then, after changing drapes, the reimplantation should be done with a “clean” table as described earlier for the one-stage reimplantation procedure. The higher cost of this setting may preclude its utilization in some hospitals; more cost-effectiveness studies should be performed to justify this practice. Six weeks of intravenous antibiotics are recommended, along with an infectious disease/medicine physician to take care of the potential complications from the long-term use of antibiotics. After reimplantation, a 3-month course of oral antibiotics specific for known microorganisms has been shown to significantly decrease the failure rate due to reinfection in 2 recent randomized controlled trials in which patients who underwent second-stage were randomly assigned to receive specific antibiotics for 3 months postoperatively or no antibiotics at all. ,
DAIR stands for d ebridement, a ntibiotics and i mplant r etention. This treatment method involves removal of the polyethylene (PE) insert or liner and all of the other modular parts; a thorough radical debridement, including the use of antiseptic solutions as described for the one-stage technique ( Table 28.3 ; mechanical and chemical disruption of the biofilm); and reinsertion of a new insert or liner, with the retention of metallic implants that are fixed to the bone. It remains an acceptable option in selected patients due to its simplicity, preservation of bone stock, lower cost, and lower morbidity. It is more beneficial for early acute infections or acute delayed infection. While this option sounds attractive, there are variable success rates that depend on factors such as timing and infecting pathogen. Simply put, DAIR is appropriate in patients with nonresistant staphylococcal infections that are addressed early—less than 7 days of drainage or symptoms. This procedure has been demonstrated to have a failure rate of as high as 84% in cases of MRSA. Unfortunately, culture results may take 48 hours or more; thus, it is believed that due to the time-sensitive nature of these outcomes, the surgeon should not wait to initiate DAIR. The decision is difficult, as it is also inappropriate to perform DAIR in patients infected by staphylococcal-resistant strains. For the frail patient whose tumor prosthesis removal may compromise the extremity, a DAIR procedure in addition to the use of chronic suppression antibiotics may be considered. These issues should be discussed with patients when DAIR is performed.
However, even with modern decision-making algorithms and surgical techniques, DAIR was only 57% successful in one recent cohort of nonstaphylococcal infections. Additionally, this study did not show a difference when the surgeon waited more than 2 days for final cultures versus taking the patient within 2 days of symptom onset. Infecting microbes was the most predictive factor of failure.
Successful outcomes from DAIR in the knee range from 20% to 75%. In a recent cohort of acute primary PJIs of the hip treated with DAIR with the addition of antibiotic beads, 8 out of 10 cases were a success. The two patients for whom the procedure failed were infected with MRSA.
Surgical Technique ,
First, skin margins should be removed and sinus tracts (if any) should be excised in the limited circumstances that DAIR is the only alternative for surgical treatment, as this technique is mostly reserved only for patients without soft-tissue compromise. This is followed by radical synovectomy and exchange of modular implants (femoral head, acetabular liner, or polyethylene insert). Then, thorough lavage and debridement is performed to remove all of the infected/necrotic tissue. In next step, the wound is soaked in povidone iodine-diluted solution for 3 minutes and irrigated with 3 L of normal saline with a pulsed lavage tool. At this point, the wound should be temporarily closed and re-draped. The new set of surgical instruments (“clean”) is used and new insert, acetabular liner, or femoral head is inserted. The wound is closed in a regular three-layer closure. A suction drain should be left in situ until there is minimal output. Occasionally, calcium phosphate beads mixed with antibiotics can be used to release a high dose of local antibiotics. Additionally, it seems that the use of intraosseous antibiotics at the time of debridement, especially on knees, may be a good adjuvant to increase success rate. If drainage persists or if the infection fails to settle, consideration has to be given to another debridement. , It is a key to use systemic antibiotics and long-term chronic suppression with oral antibiotics to decrease the failure rate.
Antibiotic Suppression Alone
Antibiotic suppression is reserved for the patient population that is unable to endure or refuses further surgery as well as having relatively low virulent bacteria amendable to oral antibiotics. This is not a first-line treatment by any means, typically appropriate only for a patient who has already undergone failed revision attempts. Patients being considered for this option need to be able to tolerate the side effects of long-term antibiotics. In a recent multivariate analysis, extended antibiotics were demonstrated as an independent predictor of treatment success, and use beyond 12 months did not add a significant benefit. Interestingly, this suppression with antibiotics is least successful when PJI is caused by S. aureus (20%) as compared with other organisms (67%). A recent publication demonstrated a 5-year infection-free survival of 68.5%.
Optimization of Spacers in Two-Stage Revisions
Cement spacers are utilized as the first stage in a two-stage procedure. Aside from providing a conduit for eluding high-dose antibiotics, they prevent soft-tissue contractures and fibrosis. The cement spacer may be articulating or static. In a 2013 systematic review of the literature concerning total knee arthroplasty revision, articulating spacers demonstrated an increased range of motion (ROM), lower reinfection rates, and lower postoperative adverse events than did static spacers, in addition to making the reimplantation surgery less complex and with lesser bone loss. Recent studies reported the same results, showing that the articulating spacers provide better ROM, UCLA activity score, Knee Society score, and functional outcomes than the static ones, , , with no significant difference in the rate of infection control and pain scores. Though not conclusive due to the absence of definitive randomized prospective studies, when controlling for case complexity, such as patients with severe bone loss, articulating spacers still showed improved infection control while providing greater ROM and reducing the number of extensile incisions and soft-tissue flaps. On the other hand, Choi et al. have shown that the satisfaction and ROM after reimplantation were not significantly different when using either articulating or static spacers in two-stage arthroplasty. The current recommendation is to use an articulating spacer whenever possible unless a static one is otherwise indicated, such as in severe bone loss (no competent metaphysis), ligamentous insufficiency, or soft-tissue compromise that prevents appropriate coverage and closure.
A static spacer for the knee can be as simple as a block of cement to hold that space. The stability of this construct can easily be increased with the use of an intramedullary nail, Steinmann pins, or cement dowel. One novel technique is using two intramedullary nails coupled together with Luque wire and placed in the femoral and tibial canal, especially when there is significant bone loss in the distal femur ( Fig. 28.1 ). The joint is held in distraction and, flexing the knee to approximately 10 degrees, the antibiotic-impregnated cement is finger packed up the metaphysis of the bones and around the nails. To facilitate spacer removal later, the spacer is put in place without the tourniquet, as the bleeding will help prevent good cementation, allowing easier future removal. The distraction of the leg is maintained until the cement hardens completely and the knee is thoroughly irrigated to avoid thermal necrosis. Usually, 2 to 3 bags of cement are sufficient to build the spacer, but larger defects may require up to 5 to 6 bags. It is recommended to use high-viscosity cement to increase the elution of antibiotics. The most common mix of antibiotics on the cement is 2 to 3 g of vancomycin in addition to 3.6 g of tobramycin per bag of cement (40 g of powder), as described previously. We also used other antibiotic powders that are thermostable depending on the culture antibiogram.