Staphylococcus aureus bacteraemia at Alice Springs Hospital, Central Australia, 2003-2006.

BACKGROUND: Infectious diseases remain the leading cause of death at Alice Springs Hospital (ASH) and Staphylococcus aureus bacteraemia (SAB) is the second most common bloodstream infection. Non-multidrug-resistant, methicillin-resistant S. aureus (nmMRSA) is endemic to the region. AIMS: To determine whether differences exist between racial groups and resistance phenotypes in the clinical manifestations and outcomes of SAB at ASH. METHODS: A retrospective review of medical and pathology records for inpatients with SAB between 1 January 2003 and 31 December 2006. RESULTS: A total of 125 patients (indigenous, 111; non-indigenous, 14) presented with SAB during the study period. Among indigenous patients, there were 95 adults and 16 children. No non-indigenous child was admitted with SAB. The mean annual incidence rate was 160.7/100 000 indigenous population and 8.1/100 000 non-indigenous population (incidence rate ratio 19.9) (P = 0.010). Isolates were predominantly methicillin-susceptible S. aureus (indigenous, 85; non-indigenous, 13). Twenty of 27 MRSA isolates were non-multidrug-resistant. Indigenous adults were more likely to present with an infective focus (indigenous, 75; non-indigenous, 6) (P = 0.004). These were most often skin infections (skin abscesses, 31; scabies, 4). Twenty-seven indigenous adults self-discharged after receiving a median of only 5 days (inter-quartile range (IQR), 3-9) of antibiotic therapy. Ninety-day mortality rates for indigenous and non-indigenous adults were 14.7% and 14.3% respectively. The median age of death for indigenous adults was 50 years (IQR, 37-68). CONCLUSIONS: Indigenous Australians have the highest reported incidence rate of SAB worldwide. This reflects the socioeconomic disadvantage experienced by indigenous Australians whose living conditions predispose to pathogen transmission and limits opportunities to maintain adequate skin hygiene.

Activity and motor unit size in partially denervated rat medial gastrocnemius.

The aim of the study was to determine whether increased motoneuron activity induced by treadmill walking would alter the extent of motoneuron sprouting in the partially denervated rat medial gastrocnemius muscle. An extensive partial denervation was effected by unilateral section of the L5 ventral root, and it is very likely that all units remaining in the medial gastrocnemius were used in treadmill walking. Rats were trained for 1.5 h/day and after 14 days were walking at least 1 km/day. Motor unit characteristics were determined 24 days after the partial denervation and were compared with units from partially denervated control (PDC) animals and with units from normal (control) animals. In PDC rats, force developed by slow, fast fatigue-resistant, and fast intermediate-fatigable motor units increased substantially compared with control animals; that of fast-fatigable units did not increase. In partially denervated exercised animals, force developed by slow and fast-fatigue-resistant units showed no further increase, but fast-intermediate- and fast-fatigable units showed significant increases compared with those in PDC animals. The changes in force were closely paralleled by changes in innervation ratios. We concluded that neuronal activity is an important factor in determining the rate of motoneuron sprouting.

Motor units within the normal rat medial gastrocnemius.

The contractile properties, fatiguability and axonal conduction velocity were determined for 118 motor units in the medial gastrocnemius of eighteen rats. Fast-twitch and slow-twitch units could be categorized on the basis of the 'sag' test. For the purposes of statistical comparison the fast-twitch units were classified on the basis of their fatiguability as fast-fatiguable (FF), fast intermediate (FI) and fast fatigue-resistant (FR). As such, FR units tended to have a longer isometric twitch time course than other fast-twitch units. On the basis of peak tetanic force FF units were largest (mean 341 +/- 120 mN) followed by FI (145 +/- 85 mN), FR (87.3 +/- 38) and slow units (40.4 +/- 11.0 mN). There were no differences in motor axonal conduction velocity. Although units were categorized, it is clear that for all characteristics investigated the fast-twitch units exist as a continuum. Some of the largest FF units (peak force 350-500 mN) failed to maintain force with high-frequency stimulation (300 Hz) and this was associated with a failure of the EMG signal.

Alterations in the contractile properties of motor units within the ageing rat medial gastrocnemius.

The decline in muscle force accompanying senescence is associated with a loss of motor nerves which provides some opportunity for collateral reinnervation and alterations in the mechanical properties of surviving motor units. The contribution of the different motor unit types to this neuromuscular reorganization was investigated by determining the isometric contractile properties of single motor units within the medial gastrocnemius of rats aged 791 +/- 39 days. These were compared with a group of similarly isolated and classified units within young adult rats aged between four and five months. The estimated number of units decreased significantly from 93 +/- 28 in young adults to 66 +/- 19 in senescent animals. There was a preferential loss of fast motor units and a significant loss of myelinated axons within the muscle nerve. The maximum tetanic force developed by FF motor units decreased with age while that of the more fatigue resistant FI and FR motor units increased. Other contractile properties were unaltered and there was no change in any contractile parameter for S unit types. Alterations in the cross-sectional areas and proportions of histochemically classified muscle fibres reflected these changes. The proportion of FG fibres declined with age and there was evidence of denervation, particularly within the peripheral rim of fast type fibres. The number of muscle fibres within the medial gastrocnemius decreased with age. These results indicate that fast motor units are most severely affected during the early stages of senescence. Within this population the FF motor units which have the largest innervation ratios in the young adult may suffer preferential degeneration.

Effect of partial denervation on motor units in the ageing rat medial gastrocnemius.

The ageing neuromuscular system is thought to undergo a continual process of reorganization as motoneurones are lost and surviving motor nerves reinnervate neighbouring denervated muscle fibres. However, the extent to which collateral reinnervation is able to compensate for neural deficits in the ageing individual is unknown. The ability of the senescent motoneurone to increase the size of its peripheral field was therefore investigated following transection of the right L5 ventral root in male Sprague Dawley rats aged 775 +/- 50 days. This procedure resulted in an extensive partial denervation of the right medial gastrocnemius muscle. After a recovery period of between 28 and 31 days the isometric contractile properties of surviving motor units were compared to control motor units from both the contralateral muscle and a group of unoperated control animals aged 791 +/- 39 days. Motor unit force was found to be unchanged after partial denervation and the absence of any alteration in motor unit size was confirmed by histological analysis. However, the time course of the isometric twitch was significantly longer for both fast and slow motor unit types and the conduction velocity of motoneurones innervating fast units was decreased following partial denervation. These results demonstrate that senescent motor nerves are unable to substantially increase the size of their peripheral fields by extensive collateral reinnervation.