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Background: Temocillin is an old antimicrobial that is resistant to hydrolysis by ESBLs but has variable activity against carbapenemase-producing Enterobacteriaceae. The current EUCAST susceptibility breakpoints for Enterobacterales are set at ≤16 mg/L (susceptible with increased exposure) based on a dose of 2 g q8h, but there is limited information on the efficacy of this dose against temocillin-susceptible carbapenemase-producing Klebsiella pneumoniae isolates., Objectives: To evaluate the efficacy of this dose using a hollow-fibre infection model (HFIM) against six KPC-2-producing clinical isolates of K. pneumoniae., Methods: The isolates were characterized by WGS and temocillin susceptibility was determined using standard and high inoculum temocillin. Mutant frequencies were estimated and temocillin activity was tested in time-kill assays and in the HFIM. At standard conditions, three of the isolates were classified as susceptible (MIC ≤ 16 mg/L) and three as resistant (MIC > 16 mg/L). The HFIM was performed over 3 days to mimic human-like pharmacokinetics of 2 g q8h. Bacterial counts were performed by plating on Mueller-Hinton agar (MHA) and MHA containing 64 mg/L temocillin to detect resistant subpopulations., Results: All isolates showed a reduction in bacterial population of at least 3 log cfu/mL within the first 8 h of simulated treatment in the hollow-fibre assay. Regrowth was observed for the three resistant isolates and one of the susceptible ones. The MIC value for these isolates was higher by at least two dilutions compared with their initial values., Conclusions: These data suggest that an optimized pharmacokinetic regimen may be of clinical interest for the treatment of KPC-2-producing K. pneumoniae susceptible to temocillin. These data showed activity of temocillin against KPC-2-producing K. pneumoniae susceptible to temocillin; however, a dose of 2g q8h administered over 30 min may be inadequate to prevent the emergence of resistant variants., (© The Author(s) 2024. Published by Oxford University Press on behalf of British Society for Antimicrobial Chemotherapy.)

Background: BaeS/BaeR is a two-component system of Escherichia coli that controls the expression of porins and efflux pumps. Its role in beta-lactam resistance is limited., Objectives: To study the role of baeS/baeR two-component system in temocillin resistance in E. coli., Methods: E. coli strain BW25113 and single-gene deletion mutants related to two-component systems were collected from the KEIO collection. Double-gen deletion mutants were generated. Temocillin-resistant mutant frequencies were determined at 32 mg/L. E. coli BW25113 mutants were selected by selective pressure from serial passages. Biological costs were analysed by growth curves. Genomes of the generated mutants were sequenced. The expression level of the mdtA, mdtB, mdtC, acrD and tolC in the ΔbaeS mutant was determined by RT-PCR (with/without temocillin exposure)., Results: The frequency of temocillin mutants ranged from 2.12 × 10-8 to 4.51 × 10-8 in single-porin mutants. No mutants were recovered from E. coli BW25113 (>10-9). Selection of temocillin-resistant variants by serial passage yielded mutants up to 128 mg/L. Mutations were found in the baeS gene. Temocillin MICs ranged from 4 to 32 mg/L (highest MICs for ΔbaeS and ΔompR). The efflux pumps mdtA, mdtB, mdtC and acrD pumps were overexpressed 3-10-fold in the presence of temocillin in ΔbaeS compared to control., Conclusions: Mutations in the sensor histidine kinase, baeS, may be involved in temocillin resistance through the expression of the efflux pumps mdtABC and acrD. In addition, the low mutation rate may be a good predictor of temocillin activity., (© The Author(s) 2024. Published by Oxford University Press on behalf of British Society for Antimicrobial Chemotherapy.)

Five patients with small-fibre neuropathy characterized by temperature-dependent spontaneous pain, hyperalgesia/allodynia and signs of neurogenic inflammation were studied clinically and thermographically, and by microneurography. Thermography revealed hyperthermia confined to painful and hyperalgesic skin of distal extremities, in absence of sympathetic vasomotor denervation. Quantitative sensory testing documented either reduced thresholds or increased suprathreshold magnitude for heat pain. Microneurography identified 13 primary cutaneous C-nociceptors generating abnormal impulses in response to electrical stimuli and, in one patient, nociceptors firing spontaneously. All five patients showed examples of double spikes, in which a single brief electrical stimulus occasionally or regularly evoked two impulses. In one case, a second impulse occurred at one of three different delays. In all five patients, warming of the skin increased the probability of a second impulse occurring. Impulse doubling has previously been reported as occurring rarely in normal subjects and is attributable to unfiltering of multiple orthodromic impulses due to unidirectional conduction failure at branch points. A higher incidence of double firing in neuropathic pain patients is probably due to a reduced safety factor for conduction in the terminal arborizations of their C-nociceptors. These observations show that unidirectional conduction block provides a peripheral mechanism of temperature-dependent nociceptor hyperactivity in small-fibre neuropathy that may contribute to hyperalgesia.

The sensory territories of different cutaneous fascicles of the superficial radial nerve were delineated by microneurography at the level of the distal forearm in humans. Three fascicular patterns were found at this level: one supplying the dorsum of the radial aspect of the dorsum of the hand over the first dorsal interosseous space; another supplying the lateral aspect of the first metacarpal extending to the lateral aspect of the thumb; and a third innervating the second interosseous space and the proximal phalanx of the index and middle fingers. The compound fascicular territory is comparable to the classical territory described for the superficial radial nerve. Intraneural microstimulation of individual fascicles did not evoke paraesthesiae or pain beyond their fascicular territory, regardless of the stimulus intensity. We conclude that the superficial radial nerve at the forearm in man is composed of only three fascicles, as shown by the present study and from previous anatomical work. Referred pain seems related to nerve activity in afferent fibres from fascicles supplying deep tissues and muscles, not from cutaneous afferents.

A syndrome of cold hyperalgesia associated with cold hypoaesthesia is described in 28 patients with peripheral polyneuropathy or mononeuropathy of various aetiologies. A mechanism of sensory disinhibition, where diminished cold-specific A delta input releases cold pain input carried by C nociceptors, is proposed to explain the hyperalgesia. In most patients, the symptomatic skin is abnormally cold. This is a likely consequence of vasospasm, due to sympathetic denervation supersensitivity, caused by dropout of sympathetic efferents as part of the small caliber nerve fibre insult. The term 'triple cold syndrome' is coined to describe this specific pathophysiological condition. Descriptively it is a mirror image of erythralgia, as described by Sir Thomas Lewis (1936) and updated by one of the present authors, a human condition also centred around anomalous primary nociceptor input, in which there is heat hyperalgesia and hot symptomatic skin due to C nociceptor sensitization and vasodilatation from antidromic discharge. Thus, like the latter condition, the triple cold syndrome emerges as an independent clinical entity with definable abnormal mechanisms which should be retrieved out of the all-embracing, descriptive, diagnostic category 'reflex sympathetic dystrophy--causalgia'.

The quantitative somatosensory thermotest (QST) assesses the function of afferent channels concerned with sensory submodalities served by small calibre fibres. Measured ramps of ascending or descending temperature are applied to the skin through a Peltier contact thermode, and detection thresholds are recorded as the subject signals the onset of a particular sensation. The present study describes underlying principles, methodological aspects and normal reference values for the QST. In patients, measurement of thresholds for cold sensation, warm sensation, cold-induced pain and heat-induced pain, applied to 465 individuals, yielded 13 abnormal patterns segregated into three main groups: (i) thermal (cold or warm) hypoaesthesia; (ii) thermal hyperalgesia (abnormally reduced threshold for cold and/or heat induced pain); (iii) thermal hypoaesthesia combined with thermal hyperalgesia. Critical analysis of these results yielded a number of observations of general relevance: (i) thermal specific (warm or cold) hypoaesthesia and thermal (heat or cold) hyperalgesia may occur in the absence of hypoaesthesia for tactile submodalities served by large calibre afferents; (ii) cold hypoaesthesia and warm hypoaesthesia may dissociate from each other; (iii) thermal pain hyperalgesias may occur in the absence of hypoaesthesias for specific cold or warm sensations; (iv) cold hyperalgesia and heat hyperalgesia may dissociate from each other. Thus, a negative routine sensory examination and unimpaired sensory nerve action potentials do not exclude possible somatosensory dysfunction. Furthermore, while most methods of sensory testing only document normality or deficit, the QST permits additional documentation of hyperalgesia, a positive sensory phenomenon that implies unusual pathophysiologies such as sensitization of receptors, central hyperexcitability, disinhibition or, possibly, ectopic nerve impulse discharge. This psychophysical test does not specify the level within afferent channels, between skin and brainmind, where the abnormality resides. It is recommended that the QST for all four thermal specific and thermal pain functions be incorporated in routine neurological assessment.

Perception thresholds for warm and cold sensation were measured by two methods, the method of levels and the method of limits, at various rates of temperature change. The following findings were obtained. (1) The threshold value is critically dependent upon the method through which it is obtained, being higher for the method that includes reaction time in the measurement. (2) When using a method that includes participation of reaction time, threshold increases with increasing rate of temperature change. (3) The artefactual threshold elevation recorded through the method of limits corresponds precisely to the reaction time. (4) Conduction velocities for the primary afferents mediating the sensations of warm and cold, calculated on the basis of reaction time and conduction distance are in keeping with the mediation of warm sensation by unmyelinated primary afferents and of cold sensation by small myelinated afferents. (5) Measurement of threshold by the method of levels and direct measurement of reaction time enables calculation of conduction velocity for the specific sensory submodality tested from a single stimulation site.

The effect of compression-ischaemia nerve block on psychophysical thresholds for warm sensation and heat-induced pain was studied on 19 normal human volunteers. Although those two sensory submodalities should be predicted to block simultaneously, based on the fact that both are served by unmyelinated primary afferents, it was actually found that warm sensation was much more vulnerable to compression-ischaemia than heat-induced pain. This is interpreted as resulting from different summation requirements for each of the two sensory modalities; sensation of warmth depends on spatial summation to a larger extent than heat-induced pain. Such differential vulnerability is in line with recent clinical studies reporting deterioration of warm sensation associated with preservation of heat pain in peripheral nerve disorders caused by diabetes, ageing and other neuropathic processes.

The peripheral distribution of the fibre content of individual ulnar nerve fascicles supplying skin and muscles of the hand in human volunteers was indirectly mapped by tracing the fields of projected sensation evoked by intraneural electrical microstimulation (INMS) and by tracing receptive fields delineated through intraneural recording of afferent impulse activity elicited by natural stimulation of end organs. Moderate intensity suprathreshold INMS, delivered in cutaneous fascicles, induced nonpainful sensations projecting to stereotyped and coherent areas of skin, the fascicular projected fields (FPFs). Fascicular receptive fields (FRFs) were mapped during microneurographic recording by determining the area of skin which, when activated by light tactile stimuli, elicited afferent neural discharges recorded intraneurally. It was found that at a given electrode position in a skin nerve fascicle, moderate intensity INMS induced nonpainful sensations projecting to a cutaneous field (FPF) that coincides with the FRF, while high intensity INMS induced painful sensations projecting within the cutaneous field of nonpainful sensations. Pain induced by INMS in muscle nerve fascicles was projected to the muscles innervated by that fascicle and, in most instances, to areas beyond the muscular receptive field. The study demonstrates that individual ulnar nerve fascicles, at wrist levels, subserve well-defined cutaneous territories in the hand, and that the area of skin covered by the sum of all ulnar fascicular receptive or projected fields matches the maximal possible cutaneous distribution of the ulnar nerve. Insights of practical relevance regarding clinical expression of fascicular nerve injuries are also brought up by this study.

While the pure sensation of cold is evoked by activation of a specific set of afferent channels, an additional set is believed to be activated by noxious low-temperature stimuli evoking cold pain. At primary afferent level, the channels concerned with the cold fraction of cold pain are served by myelinated A delta cold-specific fibres, whereas those concerned with the pain fraction are served by unmyelinated C nociceptors. In the present study, interaction between the two types of afferent input underlying cold pain was investigated by selectively blocking conduction in myelinated fibres. When doing so to the point of abolishing cold sensation, ramps of low-temperature stimuli eventually evoked a first sensation of burning pain. In addition to, and contemporaneous with, this change in quality, a significant decrease in pain threshold (reduction in required stimulus energy) was recorded when applying a noxious low-temperature stimulus. Such exaggeration in magnitude of low temperature-induced pain and the unmasking of its burning quality by A fibre block imply release of central sensory transmission due to removal of inhibitory primary afferent input. Myelinated fibres transmitting either tactile, cold sensations or both could exert this inhibition. Previous evidence of suppression of pain by low-temperature stimuli indicates that it is the cold-specific input that normally exerts this central gating on nociceptor input. The present results may also offer an explanation for the occurrence of a syndrome of burning pain on cold exposure in neuropathic patients with impaired ability to perceive cold.

Intraneural microelectrode recordings were obtained from sensory nerve fascicles in awake normal subjects experiencing post-ischaemic paraesthesiae. Spontaneous activity in afferent fibres started about half a minute after re-establishment of circulation; it peaked by the second minute and slowly waned thereafter. This parallelled the time course and intensity of the perceived paraesthesiae. During the most intense stage of paraesthesiae, single unit discharges consisted of prolonged bursts of high-frequency impulses (200-300 imp/s) lasting for 1 to 7 seconds. Repeated bursts became progressively shorter and often exhibited marked rhythmicity as paraesthesiae became weaker. Hyperventilation increased paroxysmal firing and paraesthesiae, whereas ischaemic periods repeated at short intervals exhausted the impulse generating mechanism. The outcome of local anaesthetic blocks indicated that the impulses were generated ectopically along the nerve fibres previously subjected to ischaemia and not primarily from the receptors in the skin. The high frequency of the discharge and the quality and distribution of various paraesthesiae suggest that sensory units with myelinated fibres participated in the paroxysmal discharge; normal activity, but no ectopic discharges were recorded in unmyelinated fibres. It is proposed that the perverted quality of paraesthesiae is due to an abnormal spatio-temporal pattern of ectopic impulses from different types of sensory units. creating a chaotic percept consisting of an assortment of sensations referred in irregular succession to multiple areas.

Recent advances in knowledge, and controversy, concerning cutaneous mechanoreception are reviewed. In particular, the question of perceptive specificity is discussed in the light of new experimental approaches, namely microneurography and microstimulation of identified tactile units from the glabrous skin of the human hand. Additional data are also presented. It is concluded that the human brain has an exquisite capacity to detect, localize, delineate, and classify sensations from the input of individual tactile units in the glabrous skin of the hand. The physiological specificity of low-threshold mechanoreceptors in the hand, which has been well documented in previous studies, can now be linked to distinct attributes of very simple tactile sensations which subjects report when a single afferent is stimulated electrically through an intraneural microelectrode. This conclusion does not conflict with the concept that the brain normally makes use of temporal and spatial patterns of impulses from a large number of receptors of various types to form more complex tactile percepts, such as in the recognition of texture, but the microstimulation data demonstrate that particular patterns of nerve impulses are not a necessary determinant of the subjective quality attribute of a simple tactile sensation. On the contrary, the sensations elicited by a train of impulses in a single afferent unit are remarkably distinct and well characterized in a number of respects. A further conclusion is that the afferent inputs are processed differently in tactile subsystems. This observation warrants caution in generalizing results from one system into a common theory of the origin of sensory modality.

Intraneural microstimulation combined with microneurography was employed to study the degree of mixing of muscle and skin nerve fibres at two levels in the human median nerve, and to delineate the area of skin supplied by a fascicle. It was found that about half the fascicles at upper arm level appeared to supply exclusively skin or muscle, while the remainder were considerably committed to one or the other. Further, the projection territories of skin nerve fascicles, both at the wrist and in the arm, tended to be discrete, confluent areas covering no more than 20 per cent of the median nerve territory. At the wrist fascicular territories usually resembled the innervation area of a single palmar digital nerve while in the upper arm they tended to cover a digital interspace. These findings suggest that most of the rearrangements of spinal root fibres into terminal nerve branch groupings occur at brachial plexus level, while the intraneural fascicular plexus may act as a safety mechanism allowing for nondermatomal overlap at proximal level.