Kinetics of the actions of tetracyclines on Escherichia coli as studied by microcalorimetry

In vitro antimicrobial susceptibility testing methods: agar dilution to 3D tissue-engineered models

In the field of orthopaedic surgery, bacterial invasion of implants and the resulting periprosthetic infections are a common and unresolved problem. Antimicrobial susceptibility testing methods help to define the optimal treatment and identify antimicrobial resistance. This review discusses proven gold-standard techniques and recently developed models for antimicrobial susceptibility testing, while also providing a future outlook. Conventional, gold-standard methods, such as broth microdilution, are still widely applied in clinical settings. Although recently developed methods based on microfluidics and microdroplets have shown advantages over conventional methods in terms of testing speed, safety and the potential to provide a deeper insight into resistance mechanisms, extensive validation is required to translate this research to clinical practice. Recent optical and mechanical methods are complex and expensive and, therefore, not immediately clinically applicable. Novel osteoblast infection and tissue models best resemble infections in vivo. However, the integration of biomaterials into these models remains challenging and they require a long tissue culture, making their rapid clinical implementation unlikely. A method applicable for both clinical and research environments is difficult to realise. With a continuous increase in antimicrobial resistance, there is an urgent need for methods that analyse recurrent infections to identify the optimal treatment approaches. Graphical abstract Timeline of published and partly applied antimicrobial susceptibility testing methods, listed according to their underlying mechanism, complexity and application in research or clinics.

Biomedical use of isothermal microcalorimeters

Isothermal microcalorimetry is becoming widely used for monitoring biological activities in vitro. Microcalorimeters are now able to measure heat production rates of less than a microwatt. As a result, metabolism and growth of relatively small numbers of cultured bacteria, protozoans, human cells and even small animals can be monitored continuously and extremely accurately at any chosen temperature. Dynamic effects on these organisms of changes in the culture environment—or of additions to it—are easily assessed over periods from hours to days. In addition microcalorimetry is a non-destructive method that does not require much sample preparation. It is also completely passive and thus allows subsequent evaluations of any kind on the undisturbed sample. In this review, we present a basic description of current microcalorimetry instruments and an overview of their use for various biomedical applications. These include detecting infections, evaluating effects of pharmaceutical or antimicrobial agents on cells, monitoring growth of cells harvested for tissue eingineering, and assessing medical and surgical device material physico-chemical stability and cellular biocompatibility.

In vitro activity of gallium maltolate against Staphylococci in logarithmic, stationary, and biofilm growth phases: comparison of conventional and calorimetric susceptibility testing methods

Ga(3+) is a semimetal element that competes for the iron-binding sites of transporters and enzymes. We investigated the activity of gallium maltolate (GaM), an organic gallium salt with high solubility, against laboratory and clinical strains of methicillin-susceptible Staphylococcus aureus (MSSA), methicillin-resistant S. aureus (MRSA), methicillin-susceptible Staphylococcus epidermidis (MSSE), and methicillin-resistant S. epidermidis (MRSE) in logarithmic or stationary phase and in biofilms. The MICs of GaM were higher for S. aureus (375 to 2000 microg/ml) than S. epidermidis (94 to 200 microg/ml). Minimal biofilm inhibitory concentrations were 3,000 to >or=6,000 microg/ml (S. aureus) and 94 to 3,000 microg/ml (S. epidermidis). In time-kill studies, GaM exhibited a slow and dose-dependent killing, with maximal action at 24 h against S. aureus of 1.9 log(10) CFU/ml (MSSA) and 3.3 log(10) CFU/ml (MRSA) at 3x MIC and 2.9 log(10) CFU/ml (MSSE) and 4.0 log(10) CFU/ml (MRSE) against S. epidermidis at 10x MIC. In calorimetric studies, growth-related heat production was inhibited by GaM at subinhibitory concentrations; and the minimal heat inhibition concentrations were 188 to 4,500 microg/ml (MSSA), 94 to 1,500 microg/ml (MRSA), and 94 to 375 microg/ml (MSSE and MRSE), which correlated well with the MICs. Thus, calorimetry was a fast, accurate, and simple method useful for investigation of antimicrobial activity at subinhibitory concentrations. In conclusion, GaM exhibited activity against staphylococci in different growth phases, including in stationary phase and biofilms, but high concentrations were required. These data support the potential topical use of GaM, including its use for the treatment of wound infections, MRSA decolonization, and coating of implants.

Performance of microcalorimetry for early detection of methicillin resistance in clinical isolates of Staphylococcus aureus

We describe a calorimetric assay for the detection of methicillin-resistant Staphylococcus aureus (MRSA) within 5 h. Microbial heat was calculated in culture with and without cefoxitin. Among 30 genetically distinct clinical isolates, 19/20 MRSA (95%) and 10/10 methicillin-susceptible Staphylococcus aureus (100%) were correctly identified. Microcalorimetry may be useful for rapid MRSA screening.

Suppression of tricarboxylic acid cycle in Escherichia coli exposed to sub-MICs of aminoglycosides

The metabolic activity of Escherichia coli ATCC 25922 challenged with sub-MICs of aminoglycosides was analyzed with a batch calorimeter. High-performance and gas-liquid chromatographic techniques were utilized to evaluate the concentrations of metabolic reactants, intermediates, and end products. The data reported indicate that aminoglycosides inhibit or delay bacterial catabolism of carboxylic acids, with the following relative degrees of activity: amikacin greater than gentamicin greater than sisomicin greater than netilmicin greater than kanamycin. The decrease in total biomass production was proportional to the degree of tricarboxylic acid cycle inhibition.

Antibiotic sensitivity testing by flow microcalorimetry

The proposed flow microcalorimetric method for the diagnosis of bacteriuria has been extended to include antibiotic sensitivity testing. Sensitive organisms rapidly (4-8 min) show thermal responses to the added antibiotics over the normal range of concentrations (1 x, 2x, MIC value).

Microcalorimetric study of the effects of cephalexin and cephaloridin on Escherichia coli and Staphylococcus aureus

The kinetics of the antibacterial actions of cephalexin and cephaloridin against a strain of Escherichia coli and a strain of Staphylococcus aureus were studied by a flow microcalorimeter. The heat production was related to the number of viable organisms (CFU ml-1), the pH, the optical density of the culture medium (OD540), and the morphology of the antibiotic-exposed bacteria. No heat effects could be registered when the number of CFU was below 10(4) ml-1. The addition of cephalexin, 2.5 microgram ml-1 (5 x MIC), to cultures of S. aureus caused a decrease in the heat production which was only roughly correlated with the number of CFU ml-1. This was also the case when 9.0 microgram ml-1 (2 x MIC) of this drug were added to cultures of E. coli. Two to three hours after the drugs had been added, no heat effects could be registered for the following 6--8 hours, after which an increase in the heat production again occurred. The MIC and MBC of the organisms isolated during this late heat increase were 8--40 times higher than those of the parent test organisms. A direct relation between drug concentration and response, i.e. heat effects produced, was found when increasing concentrations of cephalexin, i.e. 1.0 up to 50 micrograms ml-1 (2--100 x MIC) were added in the logarithmic growth phase to cultures of S. aureus. In ampoule calorimetric experiments, E. coli was cultured in a non-aerated, sealed growth vessel in the presence of cephalexin or cephaloridin in concentrations corresponding to 1/2 x MIC. The thermograms did not differ in shape, although the heat effects occurred somewhat later in the culture containing cephaloridin.

Microcalorimetry as a tool for evaluation of blood culture media

Evaluation of optimal compositions of blood culture media has called for extensive and laborious work in comparative studies of large series of clincal specimens. Bacterial growth is accompanied by heat production, and calorimetry provides an analytical tool for its detection and quantification. A twin microcalorimeter of the heat conduction type was used to register heat effects in experimentally infected blood cultures. When studying Escherichia coli and Staphylococcus aureus, larger heat effects were produced with 0.05% sodium polyanetholsulfonate than with 600 IU of heparin per ml, which was also the case when using 10% sucrose. The addition of IsoVitaleX (BBL) increased the heat effects produced by the two species mentioned, whereas it had the opposite effect in cultures of Neisseria meningitidis. The present study indicates that microcalorimetry is a valuable and time-saving tool for the evaluation of optimal compositions of bacterial culture media.